diff --git a/src/nvim/regexp.c b/src/nvim/regexp.c
index 8529bcb49a..4abde83bba 100644
--- a/src/nvim/regexp.c
+++ b/src/nvim/regexp.c
@@ -15525,7 +15525,30 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
           }
         }
       } else {
-        if (addstate(nextlist, start, m, NULL, clen) == NULL) {
+        uint8_t *save_line = rex.line;
+        uint8_t *save_input = rex.input;
+        linenr_T save_lnum = rex.lnum;
+
+        // At the end of a line the match can only start on the next
+        // line, use that position instead of the line break.
+        if (REG_MULTI && clen == 0 && nfa_endp != NULL
+            && rex.lnum < nfa_endp->se_u.pos.lnum) {
+          char *next_line = reg_getline(rex.lnum + 1);
+
+          if (next_line != NULL) {
+            rex.line = (uint8_t *)next_line;
+            rex.input = rex.line;
+            rex.lnum++;
+          }
+        }
+
+        r = addstate(nextlist, start, m, NULL, clen);
+
+        rex.line = save_line;
+        rex.input = save_input;
+        rex.lnum = save_lnum;
+
+        if (r == NULL) {
           nfa_match = NFA_TOO_EXPENSIVE;
           goto theend;
         }
diff --git a/test/old/testdir/test_regexp_utf8.vim b/test/old/testdir/test_regexp_utf8.vim
index d6284a559f..f5ec0b77d2 100644
--- a/test/old/testdir/test_regexp_utf8.vim
+++ b/test/old/testdir/test_regexp_utf8.vim
@@ -656,4 +656,29 @@ func Test_regex_collection_range_with_composing_crash()
   bwipe!
 endfunc
 
+" A submatch in a look-behind must not start on the previous line.  See
+" issue #20802.
+func Test_lookbehind_submatch_on_second_line()
+  new
+  for re in range(0, 2)
+    exe "set re=" .. re
+    call setline(1, ['testing', 'testing'])
+    %s/\v(.)@<=/[\1]/g
+    call assert_equal(['t[t]e[e]s[s]t[t]i[i]n[n]g',
+	  \ 't[t]e[e]s[s]t[t]i[i]n[n]g'], getline(1, '$'), 're=' .. re)
+    %d _
+
+    " With "\_." the look-behind can match the line break, then the submatch
+    " does start on the previous line.
+    call setline(1, ['abc', 'def'])
+    %s/\v(\_.)@<=/[\1]/g
+    call assert_equal(['a[a]b[b]c[c]', '[]d[d]e[e]f[f]'],
+	  \ getline(1, '$'), 're=' .. re)
+    %d _
+  endfor
+
+  set re&
+  bwipe!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab