diff --git a/app/models/import.rb b/app/models/import.rb
index 94e44c5e2..ab76e1b91 100644
--- a/app/models/import.rb
+++ b/app/models/import.rb
@@ -69,7 +69,7 @@ class Import < ApplicationRecord
     encoding = lu(user, :general_csv_encoding)
     if file_exists?
       begin
-        content = File.read(filepath, 256)
+        content = File.read(filepath, 4.kilobytes)
 
         separator = [',', ';'].max_by {|sep| content.count(sep)}
         wrapper = ['"', "'"].max_by {|quote_char| content.count(quote_char)}
diff --git a/lib/redmine/codeset_util.rb b/lib/redmine/codeset_util.rb
index aa0a23334..0d6594733 100644
--- a/lib/redmine/codeset_util.rb
+++ b/lib/redmine/codeset_util.rb
@@ -96,6 +96,12 @@ module Redmine
       return if str.nil?
 
       str = str.dup
+      # Truncate the data at the last LF character to ensure that a partial
+      # multibyte character, which could cause `String#valid_encoding?` to
+      # return false, is not included at the end of the data.
+      last_lf_index = str.rindex("\n")
+      str = str[..last_lf_index] if last_lf_index.to_i >= 64
+
       encodings = Setting.repositories_encodings.split(',').collect(&:strip)
       encodings = encodings.presence || ['UTF-8']
 
diff --git a/test/unit/lib/redmine/codeset_util_test.rb b/test/unit/lib/redmine/codeset_util_test.rb
index 39a619d4c..d71f70908 100644
--- a/test/unit/lib/redmine/codeset_util_test.rb
+++ b/test/unit/lib/redmine/codeset_util_test.rb
@@ -118,4 +118,24 @@ class Redmine::CodesetUtilTest < ActiveSupport::TestCase
       assert_nil Redmine::CodesetUtil.guess_encoding(str)
     end
   end
+
+  def test_guess_encoding_handles_trailing_partial_multibyte_character
+    str = <<~STR
+      いろはにほへと ちりぬるを
+      わかよたれそ つねならむ
+      うゐのおくやま けふこえて
+      あさきゆめみし ゑひもせす
+      色は匂へど 散りぬるを
+      我が世誰ぞ 常ならむ
+      有為の奥山 今日越えて
+      浅き夢見し 酔ひもせず
+    STR
+
+    # UTF-8 string truncated at an incomplete character boundary
+    # str.byteslice(0, 256) => "いろは...\n浅き夢見\xE3\x81"
+    # "\xE3\x81" is a part of "し" ("\xE3\x81\x97")
+    str_with_partial_char = str.byteslice(0, 256)
+    assert_not str_with_partial_char.valid_encoding?
+    assert_equal 'UTF-8', Redmine::CodesetUtil.guess_encoding(str_with_partial_char)
+  end
 end