diff --git a/cpp/src/arrow/filesystem/s3fs.cc b/cpp/src/arrow/filesystem/s3fs.cc
index 9616bac71bf9..403f2788f95c 100644
--- a/cpp/src/arrow/filesystem/s3fs.cc
+++ b/cpp/src/arrow/filesystem/s3fs.cc
@@ -271,17 +271,16 @@ Result<S3Options> S3Options::FromUri(const Uri& uri, std::string* out_path) {
     options.ConfigureDefaultCredentials();
   }
 
-  auto it = options_map.find("region");
-  if (it != options_map.end()) {
-    options.region = it->second;
-  }
-  it = options_map.find("scheme");
-  if (it != options_map.end()) {
-    options.scheme = it->second;
-  }
-  it = options_map.find("endpoint_override");
-  if (it != options_map.end()) {
-    options.endpoint_override = it->second;
+  for (const auto& kv : options_map) {
+    if (kv.first == "region") {
+      options.region = kv.second;
+    } else if (kv.first == "scheme") {
+      options.scheme = kv.second;
+    } else if (kv.first == "endpoint_override") {
+      options.endpoint_override = kv.second;
+    } else {
+      return Status::Invalid("Unexpected query parameter in S3 URI: '", kv.first, "'");
+    }
   }
 
   return options;
diff --git a/cpp/src/arrow/filesystem/s3fs_test.cc b/cpp/src/arrow/filesystem/s3fs_test.cc
index 9fd1b57cdc18..ac9fb31648e3 100644
--- a/cpp/src/arrow/filesystem/s3fs_test.cc
+++ b/cpp/src/arrow/filesystem/s3fs_test.cc
@@ -218,6 +218,9 @@ TEST_F(S3OptionsTest, FromUri) {
 
   // Missing bucket name
   ASSERT_RAISES(Invalid, S3Options::FromUri("s3:///foo/bar/", &path));
+
+  // Invalid option
+  ASSERT_RAISES(Invalid, S3Options::FromUri("s3://mybucket/?xxx=zzz", &path));
 }
 
 TEST_F(S3OptionsTest, FromAccessKey) {
diff --git a/cpp/src/arrow/util/uri.cc b/cpp/src/arrow/util/uri.cc
index b116a1362776..795e3fa2c8b0 100644
--- a/cpp/src/arrow/util/uri.cc
+++ b/cpp/src/arrow/util/uri.cc
@@ -49,12 +49,21 @@ std::string TextRangeToString(const UriTextRangeStructA& range) {
 bool IsTextRangeSet(const UriTextRangeStructA& range) { return range.first != nullptr; }
 
 #ifdef _WIN32
-bool IsDriveSpec(util::string_view s) {
+bool IsDriveSpec(const util::string_view s) {
   return (s.length() >= 2 && s[1] == ':' &&
           ((s[0] >= 'A' && s[0] <= 'Z') || (s[0] >= 'a' && s[0] <= 'z')));
 }
 #endif
 
+std::string UriUnescape(const util::string_view s) {
+  std::string result(s);
+  if (!result.empty()) {
+    auto end = uriUnescapeInPlaceA(&result[0]);
+    result.resize(end - &result[0]);
+  }
+  return result;
+}
+
 }  // namespace
 
 std::string UriEscape(const std::string& s) {
@@ -125,9 +134,9 @@ std::string Uri::username() const {
   auto userpass = TextRangeToView(impl_->uri_.userInfo);
   auto sep_pos = userpass.find_first_of(':');
   if (sep_pos == util::string_view::npos) {
-    return std::string(userpass);
+    return UriUnescape(userpass);
   } else {
-    return std::string(userpass.substr(0, sep_pos));
+    return UriUnescape(userpass.substr(0, sep_pos));
   }
 }
 
@@ -137,7 +146,7 @@ std::string Uri::password() const {
   if (sep_pos == util::string_view::npos) {
     return std::string();
   } else {
-    return std::string(userpass.substr(sep_pos + 1));
+    return UriUnescape(userpass.substr(sep_pos + 1));
   }
 }
 
diff --git a/cpp/src/arrow/util/uri_test.cc b/cpp/src/arrow/util/uri_test.cc
index f4d5d078ff46..44804704c436 100644
--- a/cpp/src/arrow/util/uri_test.cc
+++ b/cpp/src/arrow/util/uri_test.cc
@@ -212,6 +212,19 @@ TEST(Uri, ParseUserPass) {
   ASSERT_EQ(uri.host(), "localhost");
   ASSERT_EQ(uri.username(), "someuser");
   ASSERT_EQ(uri.password(), "somepass");
+
+  // With %-encoding
+  ASSERT_OK(uri.Parse("http://some%20user%2Fname:somepass@localhost"));
+  ASSERT_EQ(uri.scheme(), "http");
+  ASSERT_EQ(uri.host(), "localhost");
+  ASSERT_EQ(uri.username(), "some user/name");
+  ASSERT_EQ(uri.password(), "somepass");
+
+  ASSERT_OK(uri.Parse("http://some%20user%2Fname:some%20pass%2Fword@localhost"));
+  ASSERT_EQ(uri.scheme(), "http");
+  ASSERT_EQ(uri.host(), "localhost");
+  ASSERT_EQ(uri.username(), "some user/name");
+  ASSERT_EQ(uri.password(), "some pass/word");
 }
 
 TEST(Uri, FileScheme) {