diff --git a/runtime/doc/api.txt b/runtime/doc/api.txt
index 318ba96605..3813792418 100644
--- a/runtime/doc/api.txt
+++ b/runtime/doc/api.txt
@@ -3921,9 +3921,11 @@ nvim_open_win({buffer}, {enter}, {config})                   *nvim_open_win()*
                     Default is `"left"`.
                   • vertical: Split vertically |:vertical|.
                   • width: Window width (in character cells). Minimum of 1.
-                  • win: |window-ID| window to split, or relative window when
-                    creating a float (relative="win"). When splitting,
-                    negative value works like |:topleft|, |:botright|.
+                  • win: |window-ID| target window. Can be in a different tab
+                    page. Determines the window to split (negative values act
+                    like |:topleft|, |:botright|), the relative window for a
+                    `relative="win"` float, or just the target tab page
+                    (inferred from the window) for others.
                   • zindex: Stacking order. floats with higher `zindex` go on
                     top on floats with lower indices. Must be larger than
                     zero. The following screen elements have hard-coded
diff --git a/runtime/lua/vim/_meta/api.lua b/runtime/lua/vim/_meta/api.lua
index 975db4ac3b..0510e2bef9 100644
--- a/runtime/lua/vim/_meta/api.lua
+++ b/runtime/lua/vim/_meta/api.lua
@@ -1848,8 +1848,9 @@ function vim.api.nvim_open_term(buffer, opts) end
 ---     Default is `"left"`.
 --- - vertical: Split vertically `:vertical`.
 --- - width: Window width (in character cells). Minimum of 1.
---- - win: `window-ID` window to split, or relative window when creating a float (relative="win").
----     When splitting, negative value works like `:topleft`, `:botright`.
+--- - win: `window-ID` target window. Can be in a different tab page. Determines the window to
+---     split (negative values act like `:topleft`, `:botright`), the relative window for a
+---     `relative="win"` float, or just the target tab page (inferred from the window) for others.
 --- - zindex: Stacking order. floats with higher `zindex` go on top on
 ---             floats with lower indices. Must be larger than zero. The
 ---             following screen elements have hard-coded z-indices:
diff --git a/src/nvim/api/win_config.c b/src/nvim/api/win_config.c
index 5c7f3a7d32..cf10883d81 100644
--- a/src/nvim/api/win_config.c
+++ b/src/nvim/api/win_config.c
@@ -180,8 +180,9 @@
 ///       Default is `"left"`.
 ///   - vertical: Split vertically |:vertical|.
 ///   - width: Window width (in character cells). Minimum of 1.
-///   - win: |window-ID| window to split, or relative window when creating a float (relative="win").
-///       When splitting, negative value works like |:topleft|, |:botright|.
+///   - win: |window-ID| target window. Can be in a different tab page. Determines the window to
+///       split (negative values act like |:topleft|, |:botright|), the relative window for a
+///       `relative="win"` float, or just the target tab page (inferred from the window) for others.
 ///   - zindex: Stacking order. floats with higher `zindex` go on top on
 ///               floats with lower indices. Must be larger than zero. The
 ///               following screen elements have hard-coded z-indices:
@@ -745,6 +746,16 @@ void nvim_win_set_config(Window window, Dict(win_config) *config, Error *err)
   }
 
   if (was_split && !to_split) {
+    win_T *parent = find_window_by_handle(fconfig.window, err);
+    if (!parent) {
+      return;
+    }
+    // TODO(seandewar): support this, preferably via win_config_float_tp.
+    if (!win_valid(parent)) {
+      api_set_error(err, kErrorTypeValidation,
+                    "Cannot configure split into float in another tabpage");
+      return;
+    }
     if (!win_new_float(win, false, fconfig, err)) {
       return;
     }
@@ -1351,33 +1362,31 @@ static bool parse_win_config(win_T *wp, Dict(win_config) *config, WinConfig *fco
     goto fail;
   }
 
-  if (relative_is_win || is_split) {
-    if (wp && relative_is_win) {
-      win_T *target_win = find_window_by_handle(config->win, err);
-      if (!target_win) {
-        goto fail;
-      }
-
-      if (target_win == wp) {
-        api_set_error(err, kErrorTypeException, "floating window cannot be relative to itself");
-        goto fail;
-      }
+  if (relative_is_win || (HAS_KEY_X(config, win) && !is_split && wp && wp->w_floating
+                          && fconfig->relative == kFloatRelativeWindow)) {
+    // When relative=win is given, missing win field means win=0.
+    win_T *target_win = find_window_by_handle(config->win, err);
+    if (!target_win) {
+      goto fail;
     }
-    fconfig->window = curwin->handle;
+    if (target_win == wp) {
+      api_set_error(err, kErrorTypeException, "floating window cannot be relative to itself");
+      goto fail;
+    }
+    fconfig->window = target_win->handle;
+  } else {
+    // Handle is not validated here, as win_config_split can accept negative values.
     if (HAS_KEY_X(config, win)) {
-      if (config->win > 0) {
-        fconfig->window = config->win;
+      if (!is_split && !has_relative && (!wp || !wp->w_floating)) {
+        api_set_error(err, kErrorTypeValidation,
+                      "non-float with 'win' requires at least 'split' or 'vertical'");
+        goto fail;
       }
+      fconfig->window = config->win;
     }
-  } else if (HAS_KEY_X(config, win)) {
-    if (has_relative) {
-      api_set_error(err, kErrorTypeValidation,
-                    "'win' key is only valid with relative='win' and relative=''");
-      goto fail;
-    } else if (!is_split) {
-      api_set_error(err, kErrorTypeValidation,
-                    "non-float with 'win' requires at least 'split' or 'vertical'");
-      goto fail;
+    // Resolve, but skip validating. E.g: win_config_split accepts negative "win".
+    if (fconfig->window == 0) {
+      fconfig->window = curwin->handle;
     }
   }
 
diff --git a/test/functional/api/window_spec.lua b/test/functional/api/window_spec.lua
index 1c48c51921..579eb88663 100644
--- a/test/functional/api/window_spec.lua
+++ b/test/functional/api/window_spec.lua
@@ -2585,19 +2585,26 @@ describe('API/win', function()
         },
       }, layout)
 
+      -- converting split into a float for a different tabpage is not yet supported
+      eq(
+        'Cannot configure split into float in another tabpage',
+        pcall_err(
+          api.nvim_win_set_config,
+          win,
+          { relative = 'editor', row = 0, col = 0, width = 1, height = 1, win = first_win }
+        )
+      )
+
       -- convert new win to float in new tabpage
       api.nvim_win_set_config(win, { relative = 'editor', row = 2, col = 2, height = 2, width = 2 })
       api.nvim_set_current_tabpage(first_tab)
       -- move to other tabpage
-      api.nvim_win_set_config(win, { relative = 'win', win = first_win, row = 2, col = 2 })
+      api.nvim_win_set_config(win, { win = first_win })
       eq(first_tab, api.nvim_win_get_tabpage(win))
       eq({ first_win, win }, api.nvim_tabpage_list_wins(first_tab))
       eq({ tab2_win }, api.nvim_tabpage_list_wins(new_tab))
       -- unlike splits, negative win is invalid
-      eq(
-        'Invalid window id: -1',
-        pcall_err(api.nvim_win_set_config, win, { relative = 'win', win = -1, row = 2, col = 2 })
-      )
+      eq('Invalid window id: -1', pcall_err(api.nvim_win_set_config, win, { win = -1 }))
     end)
 
     it('correctly moves curwin when moving curwin to a different tabpage', function()
@@ -2875,7 +2882,7 @@ describe('API/win', function()
       })
       api.nvim_set_current_tabpage(tab1)
       api.nvim_set_var('result', {})
-      api.nvim_win_set_config(fwin, { relative = 'win', win = tab1_win1, row = 4, col = 4 })
+      api.nvim_win_set_config(fwin, { win = tab1_win1 })
       eq({}, eval('g:result'))
     end)
 
@@ -3584,11 +3591,7 @@ describe('API/win', function()
           split_ok, split_err =
             pcall(vim.api.nvim_win_set_config, 0, { win = other_tp_win, split = 'right' })
 
-          float_ok, float_err = pcall(
-            vim.api.nvim_win_set_config,
-            0,
-            { relative = 'win', win = other_tp_win, row = 0, col = 0 }
-          )
+          float_ok, float_err = pcall(vim.api.nvim_win_set_config, 0, { win = other_tp_win })
         end)
         return win_type, split_ok, split_err, float_ok, float_err
       end)
@@ -3714,11 +3717,7 @@ describe('API/win', function()
       command('autocmd WinLeave * ++once call nvim_win_close(' .. tab2_win .. ', v:true)')
       eq(
         'Target windows were closed',
-        pcall_err(
-          api.nvim_win_set_config,
-          float_win,
-          { relative = 'win', win = tab2_win, row = 0, col = 0 }
-        )
+        pcall_err(api.nvim_win_set_config, float_win, { win = tab2_win })
       )
       eq(float_win, api.nvim_get_current_win())
 
@@ -3727,11 +3726,7 @@ describe('API/win', function()
       command('tabprev | autocmd WinEnter * ++once wincmd p')
       eq(
         ('Failed to switch away from window %d'):format(float_win),
-        pcall_err(
-          api.nvim_win_set_config,
-          float_win,
-          { relative = 'win', win = tab3_win, row = 0, col = 0 }
-        )
+        pcall_err(api.nvim_win_set_config, float_win, { win = tab3_win })
       )
       eq(float_win, api.nvim_get_current_win())
 
@@ -3743,11 +3738,7 @@ describe('API/win', function()
       )
       eq(
         ('Window %d was made non-floating'):format(float_win),
-        pcall_err(
-          api.nvim_win_set_config,
-          float_win,
-          { relative = 'win', win = tab3_win, row = 0, col = 0 }
-        )
+        pcall_err(api.nvim_win_set_config, float_win, { win = tab3_win })
       )
       eq(float_win, api.nvim_get_current_win())
 
@@ -3776,11 +3767,7 @@ describe('API/win', function()
       )
       eq(
         ('Cannot move external window to another tabpage'):format(float_win),
-        pcall_err(
-          api.nvim_win_set_config,
-          float_win,
-          { relative = 'win', win = tab3_win, row = 0, col = 0 }
-        )
+        pcall_err(api.nvim_win_set_config, float_win, { win = tab3_win })
       )
       eq(float_win, api.nvim_get_current_win())
     end)
diff --git a/test/functional/ui/float_spec.lua b/test/functional/ui/float_spec.lua
index 1c7f309627..a3d4289520 100644
--- a/test/functional/ui/float_spec.lua
+++ b/test/functional/ui/float_spec.lua
@@ -3783,8 +3783,8 @@ describe('float window', function()
       local buf = api.nvim_create_buf(false, false)
       eq("Invalid key: 'bork'", pcall_err(api.nvim_open_win, buf, false, { width = 20, height = 2, bork = true }))
       eq(
-        "'win' key is only valid with relative='win' and relative=''",
-        pcall_err(api.nvim_open_win, buf, false, { width = 20, height = 2, relative = 'editor', row = 0, col = 0, win = 0 })
+        "Must specify 'relative' or 'external' when creating a float",
+        pcall_err(api.nvim_open_win, buf, false, { win = 0 })
       )
       eq(
         "floating windows cannot have 'vertical'",
@@ -11191,7 +11191,17 @@ describe('float window', function()
       local winid = api.nvim_open_win(buf, false, config)
       api.nvim_set_current_win(winid)
       eq('floating window cannot be relative to itself', pcall_err(api.nvim_win_set_config, winid, config))
-      -- Also when configuring split into float.
+      eq('floating window cannot be relative to itself', pcall_err(api.nvim_win_set_config, winid, { win = winid }))
+      -- Don't assume win=0 if no win given for existing relative=win float; so no error.
+      api.nvim_win_set_config(winid, { width = 7 })
+      eq(7, api.nvim_win_get_config(winid).width)
+      -- Don't expect the error when configuring to something other than relative=win, as win=self
+      -- is fine in those cases. (though maybe pointless) Other errors might be expected, though.
+      eq('Cannot split a floating window', pcall_err(api.nvim_win_set_config, winid, { split = 'above', win = winid }))
+      eq('win', api.nvim_win_get_config(winid).relative)
+      api.nvim_win_set_config(winid, { relative = 'editor', win = winid, row = 3, col = 3 })
+      eq('editor', api.nvim_win_get_config(winid).relative)
+      -- An error when configuring split into relative=win float.
       command('split')
       eq('floating window cannot be relative to itself', pcall_err(api.nvim_win_set_config, 0, config))
     end)
@@ -11881,9 +11891,9 @@ describe('float window', function()
       -- Schedule an UPD_NOT_VALID redraw, but in one event move the float out of curtab before it's
       -- handled. Do not flush before then.
       local float = exec_lua(function()
-        local float = vim.api.nvim_open_win(0, true, { relative = 'editor', width = 10, height = 5, row = 0, col = 0 })
+        local float = vim.api.nvim_open_win(0, true, { relative = 'editor', width = 10, height = 5, row = 1, col = 1 })
         vim.api.nvim__redraw({ valid = false, flush = false })
-        vim.api.nvim_win_set_config(float, { relative = 'win', win = tab1_win, row = 1, col = 1 })
+        vim.api.nvim_win_set_config(float, { win = tab1_win })
         return float
       end)
 
@@ -11914,7 +11924,7 @@ describe('float window', function()
       end
 
       -- Importantly, want tabline redrawn and float's hl attribs to be correct here.
-      api.nvim_win_set_config(float, { relative = 'win', win = 0, row = 1, col = 1 })
+      api.nvim_win_set_config(float, { win = 0 })
       if multigrid then
         screen:expect({
           grid = [[
@@ -11935,7 +11945,7 @@ describe('float window', function()
             {2:~         }|*4
           ]],
           float_pos = {
-            [5] = { 1002, 'NW', 4, 1, 1, true, 50, 1, 1, 1 },
+            [5] = { 1002, 'NW', 1, 1, 1, true, 50, 1, 1, 1 },
           },
         })
       else
@@ -12053,7 +12063,7 @@ describe('float window', function()
         ]])
       end
 
-      api.nvim_win_set_config(float, { relative = 'win', win = tab2_win, row = 1, col = 1 })
+      api.nvim_win_set_config(float, { win = tab2_win })
       if multigrid then
         screen:expect({
           grid = [[