diff --git a/lib/Parse/Lexer.cpp b/lib/Parse/Lexer.cpp
index 1caa40abc68b4..9ef984d848057 100644
--- a/lib/Parse/Lexer.cpp
+++ b/lib/Parse/Lexer.cpp
@@ -1843,9 +1843,29 @@ void Lexer::lexStringLiteral(unsigned CustomDelimiterLen) {
   assert((QuoteChar == '"' || QuoteChar == '\'') && "Unexpected start");
 
   bool IsMultilineString = advanceIfMultilineDelimiter(CurPtr, Diags);
-  if (IsMultilineString && *CurPtr != '\n' && *CurPtr != '\r')
-    diagnose(CurPtr, diag::lex_illegal_multiline_string_start)
-        .fixItInsert(Lexer::getSourceLoc(CurPtr), "\n");
+  if (IsMultilineString && *CurPtr != '\n' && *CurPtr != '\r') {
+    // Test for single-line string literals that may resemble multiline delimiter.
+    if (CustomDelimiterLen > 0) {
+      for (const char *Ptr = CurPtr-1; Ptr <= BufferEnd-CustomDelimiterLen; Ptr++) {
+        if (*Ptr == '\r' || *Ptr == '\n') {
+          break;
+        }
+        if (*Ptr == '"') {
+          const char *TmpPtr = Ptr + 1;
+          if (delimiterMatches(CustomDelimiterLen, TmpPtr, nullptr)) {
+            // Undo effects from falsely detecting multiline delimiter.
+            CurPtr = CurPtr - 2;
+            IsMultilineString = false;
+            break;
+          }
+        }
+      }
+    }
+    if (IsMultilineString) {
+      diagnose(CurPtr, diag::lex_illegal_multiline_string_start)
+          .fixItInsert(Lexer::getSourceLoc(CurPtr), "\n");
+    }
+  }
 
   bool wasErroneous = false;
   while (true) {
diff --git a/test/Parse/raw_string.swift b/test/Parse/raw_string.swift
index d07bbd24fa54f..b84ebef7f94f7 100644
--- a/test/Parse/raw_string.swift
+++ b/test/Parse/raw_string.swift
@@ -68,6 +68,18 @@ _ = ##"""
       """##
 // CHECK: "a raw string with \"\"\" in it"
 
+_ = #"""#
+// CHECK: "\""
+
+_ = ##""" foo # "# "##
+// CHECK: "\"\" foo # \"# "
+
+_ = #"""""#
+// CHECK: "\"\"\""
+
+_ = ###"""##"###
+// CHECK: "\"\"##"
+
 let foo = "Interpolation"
 _ = #"\b\b \#(foo)\#(foo) Kappa"#
 // CHECK: "\\b\\b "
diff --git a/test/Parse/raw_string_errors.swift b/test/Parse/raw_string_errors.swift
index 0ba693fc79f30..145e082bfa109 100644
--- a/test/Parse/raw_string_errors.swift
+++ b/test/Parse/raw_string_errors.swift
@@ -12,13 +12,23 @@ let _ = #"\##("invalid")"#
 let _ = ####"invalid"###
 // expected-error@-1{{unterminated string literal}}
 
+let _ = ###"""invalid"######
+// expected-error@-1{{too many '#' characters in closing delimiter}}{{26-29=}}
+// expected-error@-2{{consecutive statements on a line must be separated by ';'}}
+// expected-error@-3{{expected expression}}
+
 let _ = ###"invalid"######
 // expected-error@-1{{too many '#' characters in closing delimiter}}{{24-27=}}
 // expected-error@-2{{consecutive statements on a line must be separated by ';'}}
 // expected-error@-3{{expected expression}}
 
-let _ = ##"""##
+let _ = ##"""a
   foobar
-  ##"""##
-// expected-error@-3{{multi-line string literal content must begin on a new line}}{{14-14=\n}}
+  a"""##
+// expected-error@-1{{multi-line string literal content must begin on a new line}}{{14-14=\n}}
 // expected-error@-2{{multi-line string literal closing delimiter must begin on a new line}}{{5-5=\n}}
+
+let _ = #""" foo "bar" #baz
+"""#
+// expected-error@-1{{multi-line string literal content must begin on a new line}}{{14-14=\n}}
+