diff --git a/AK/MemoryStream.cpp b/AK/MemoryStream.cpp
index ad4967f370..880d8027c9 100644
--- a/AK/MemoryStream.cpp
+++ b/AK/MemoryStream.cpp
@@ -81,10 +81,10 @@ ErrorOr<size_t> FixedMemoryStream::seek(i64 offset, SeekMode seek_mode)
         m_offset += offset;
         break;
     case SeekMode::FromEndPosition:
-        if (offset > static_cast<i64>(m_bytes.size()))
+        if (-offset > static_cast<i64>(m_bytes.size()))
             return Error::from_string_view_or_print_error_and_return_errno("Offset past the start of the stream memory"sv, EINVAL);
 
-        m_offset = m_bytes.size() - offset;
+        m_offset = m_bytes.size() + offset;
         break;
     }
     return m_offset;
diff --git a/Tests/AK/TestMemoryStream.cpp b/Tests/AK/TestMemoryStream.cpp
index 8d7e5ae42f..a945bb3cbb 100644
--- a/Tests/AK/TestMemoryStream.cpp
+++ b/Tests/AK/TestMemoryStream.cpp
@@ -151,3 +151,99 @@ TEST_CASE(allocating_memory_stream_offset_of_with_write_offset_multiple_of_chunk
         EXPECT_EQ(offset.value(), AllocatingMemoryStream::CHUNK_SIZE - 32 - 1);
     }
 }
+
+TEST_CASE(fixed_memory_read_write)
+{
+    constexpr auto some_words = "These are some words"sv;
+
+    auto empty = TRY_OR_FAIL(ByteBuffer::create_uninitialized(some_words.length()));
+    FixedMemoryStream stream { empty.bytes() };
+
+    ReadonlyBytes buffer { some_words.characters_without_null_termination(), some_words.length() };
+    TRY_OR_FAIL(stream.write_some(buffer));
+
+    EXPECT_EQ(TRY_OR_FAIL(stream.tell()), some_words.length());
+    EXPECT(stream.is_eof());
+
+    TRY_OR_FAIL(stream.seek(0));
+    auto contents = TRY_OR_FAIL(stream.read_until_eof());
+    EXPECT_EQ(contents.bytes(), some_words.bytes());
+}
+
+TEST_CASE(fixed_memory_close)
+{
+    auto empty = TRY_OR_FAIL(ByteBuffer::create_uninitialized(64));
+    FixedMemoryStream stream { empty.bytes() };
+    EXPECT(stream.is_open());
+    stream.close();
+    EXPECT(stream.is_open());
+}
+
+TEST_CASE(fixed_memory_read_only)
+{
+    constexpr auto some_words = "These are some words"sv;
+
+    FixedMemoryStream stream { ReadonlyBytes { some_words.bytes() } };
+
+    auto contents = TRY_OR_FAIL(stream.read_until_eof());
+    EXPECT_EQ(contents.bytes(), some_words.bytes());
+
+    TRY_OR_FAIL(stream.seek(0));
+    ReadonlyBytes buffer { some_words.characters_without_null_termination(), some_words.length() };
+    EXPECT_CRASH("Write protection assert", [&] {
+        (void)stream.write_some(buffer);
+        return Test::Crash::Failure::DidNotCrash;
+    });
+
+    EXPECT_EQ(TRY_OR_FAIL(stream.tell()), 0ull);
+    EXPECT(!stream.is_eof());
+}
+
+TEST_CASE(fixed_memory_seeking_around)
+{
+    auto stream_buffer = TRY_OR_FAIL(ByteBuffer::create_uninitialized(8702ul));
+    FixedMemoryStream stream { ReadonlyBytes { stream_buffer.bytes() } };
+
+    auto buffer = TRY_OR_FAIL(ByteBuffer::create_uninitialized(16));
+
+    TRY_OR_FAIL(stream.seek(500, SeekMode::SetPosition));
+    EXPECT_EQ(stream.tell().release_value(), 500ul);
+    TRY_OR_FAIL(stream.read_until_filled(buffer));
+
+    TRY_OR_FAIL(stream.seek(234, SeekMode::FromCurrentPosition));
+    EXPECT_EQ(stream.tell().release_value(), 750ul);
+    TRY_OR_FAIL(stream.read_until_filled(buffer));
+
+    TRY_OR_FAIL(stream.seek(-105, SeekMode::FromEndPosition));
+    EXPECT_EQ(stream.tell().release_value(), 8597ul);
+    TRY_OR_FAIL(stream.read_until_filled(buffer));
+}
+
+BENCHMARK_CASE(fixed_memory_tell)
+{
+    auto stream_buffer = TRY_OR_FAIL(ByteBuffer::create_uninitialized(10 * KiB));
+    FixedMemoryStream stream { ReadonlyBytes { stream_buffer.bytes() } };
+
+    auto expected_fixed_memory_offset = 0u;
+    auto ten_byte_buffer = TRY_OR_FAIL(ByteBuffer::create_uninitialized(1));
+    for (auto i = 0u; i < 4000; ++i) {
+        TRY_OR_FAIL(stream.read_until_filled(ten_byte_buffer));
+        expected_fixed_memory_offset += 1u;
+        EXPECT_EQ(expected_fixed_memory_offset, TRY_OR_FAIL(stream.tell()));
+    }
+
+    for (auto i = 0u; i < 4000; ++i) {
+        auto seek_fixed_memory_offset = TRY_OR_FAIL(stream.seek(-1, SeekMode::FromCurrentPosition));
+        expected_fixed_memory_offset -= 1;
+        EXPECT_EQ(seek_fixed_memory_offset, TRY_OR_FAIL(stream.tell()));
+        EXPECT_EQ(expected_fixed_memory_offset, TRY_OR_FAIL(stream.tell()));
+    }
+}
+
+TEST_CASE(fixed_memory_truncate)
+{
+    auto stream_buffer = TRY_OR_FAIL(ByteBuffer::create_uninitialized(10 * KiB));
+    FixedMemoryStream stream { ReadonlyBytes { stream_buffer.bytes() } };
+
+    EXPECT(stream.truncate(999).is_error());
+}