diff --git a/lib/net/imap.rb b/lib/net/imap.rb
index 02731bbe1..ec3c2479f 100644
--- a/lib/net/imap.rb
+++ b/lib/net/imap.rb
@@ -1949,12 +1949,15 @@ def ensure_mod_sequence_value(num)
 
     # Net::IMAP::ResponseCode represents response codes.
     #
-    #   resp_text_code  ::= "ALERT" / "PARSE" /
-    #                       "PERMANENTFLAGS" SPACE "(" #(flag / "\*") ")" /
+    #   resp_text_code  ::= "ALERT" /
+    #                       "BADCHARSET" [SP "(" astring *(SP astring) ")" ] /
+    #                       capability_data / "PARSE" /
+    #                       "PERMANENTFLAGS" SP "("
+    #                       [flag_perm *(SP flag_perm)] ")" /
     #                       "READ-ONLY" / "READ-WRITE" / "TRYCREATE" /
-    #                       "UIDVALIDITY" SPACE nz_number /
-    #                       "UNSEEN" SPACE nz_number /
-    #                       atom [SPACE 1*<any TEXT_CHAR except "]">]
+    #                       "UIDNEXT" SP nz_number / "UIDVALIDITY" SP nz_number /
+    #                       "UNSEEN" SP nz_number /
+    #                       atom [SP 1*<any TEXT-CHAR except "]">]
     #
     # ==== Fields:
     #
@@ -3450,12 +3453,25 @@ def resp_text
         end
       end
 
+      # See https://www.rfc-editor.org/errata/rfc3501
+      #
+      # resp-text-code  = "ALERT" /
+      #                   "BADCHARSET" [SP "(" charset *(SP charset) ")" ] /
+      #                   capability-data / "PARSE" /
+      #                   "PERMANENTFLAGS" SP "("
+      #                   [flag-perm *(SP flag-perm)] ")" /
+      #                   "READ-ONLY" / "READ-WRITE" / "TRYCREATE" /
+      #                   "UIDNEXT" SP nz-number / "UIDVALIDITY" SP nz-number /
+      #                   "UNSEEN" SP nz-number /
+      #                   atom [SP 1*<any TEXT-CHAR except "]">]
       def resp_text_code
         token = match(T_ATOM)
         name = token.value.upcase
         case name
         when /\A(?:ALERT|PARSE|READ-ONLY|READ-WRITE|TRYCREATE|NOMODSEQ)\z/n
           result = ResponseCode.new(name, nil)
+        when /\A(?:BADCHARSET)\z/n
+          result = ResponseCode.new(name, charset_list)
         when /\A(?:CAPABILITY)\z/ni
           result = ResponseCode.new(name, capability_data)
         when /\A(?:PERMANENTFLAGS)\z/n
@@ -3477,6 +3493,19 @@ def resp_text_code
         return result
       end
 
+      def charset_list
+        result = []
+        if accept(T_SPACE)
+          match(T_LPAR)
+          result << charset
+          while accept(T_SPACE)
+            result << charset
+          end
+          match(T_RPAR)
+        end
+        result
+      end
+
       def address_list
         token = lookahead
         if token.symbol == T_NIL
@@ -3637,6 +3666,17 @@ def combine_adjacent(*tokens)
         result
       end
 
+      # See https://www.rfc-editor.org/errata/rfc3501
+      #
+      # charset = atom / quoted
+      def charset
+        if token = accept(T_QUOTED)
+          token.value
+        else
+          atom
+        end
+      end
+
       def number
         token = lookahead
         if token.symbol == T_NIL
diff --git a/test/net/imap/test_imap_response_parser.rb b/test/net/imap/test_imap_response_parser.rb
index a059c6ce8..5b519edef 100644
--- a/test/net/imap/test_imap_response_parser.rb
+++ b/test/net/imap/test_imap_response_parser.rb
@@ -330,6 +330,14 @@ def test_msg_rfc3501_response_text_with_T_LBRA
     assert_equal("[Gmail]/Sent Mail selected. (Success)", response.data.text)
   end
 
+  def test_msg_rfc3501_response_text_with_BADCHARSET_astrings
+    parser = Net::IMAP::ResponseParser.new
+    response = parser.parse("t BAD [BADCHARSET (US-ASCII \"[astring with brackets]\")] unsupported charset foo.\r\n")
+    assert_equal("t", response.tag)
+    assert_equal("unsupported charset foo.", response.data.text)
+    assert_equal("BADCHARSET", response.data.code.name)
+  end
+
   def test_continuation_request_without_response_text
     parser = Net::IMAP::ResponseParser.new
     response = parser.parse("+\r\n")