Magnifier, formerly Microsoft Magnifier, is a screen magnifier app intended for visually impaired people to use when running Microsoft Windows. When it is running, it creates a bar at the top of the screen that greatly magnifies where the mouse is. Magnifier was first included as a sample in the Active Accessibility SDK/RDK for Windows 95 and later made a standard Windows utility starting with Windows 98. Prior to Windows Vista, Magnifier could be used to magnify the screen up to 9 times its normal size. Windows Vista and later allow up to 16× magnification.

In Windows Vista, Magnifier uses WPF, which in turn uses vector images to render the content. As a result, the rendered magnified image is sharp and not pixelated. However, this is useful only for Windows Presentation Foundation applications. Non-WPF applications are still magnified the traditional way. Also, due to a change introduced in WPF 3.5 SP1, this functionality is lost if .NET Framework 3.5 SP1 is installed.

Microsoft has also released a Magnification API to allow assistive technology applications to use the Magnifier engine.

Microsoft's Windows 7 operating system includes a significantly improved version of Magnifier. It features full-screen magnification which allows a user to pan around the screen at up to 16× magnification.

However, the full screen feature has also been criticized due to its incompatibility with the high contrast color schemes found in the Windows 7 beta release. This issue remains in the final Windows 7 release. Besides this, when the magnifier zooms, the text will appear blurry or pixelated because it is not being directly rendered at the larger size; instead, the smaller sized rendering is being enlarged as a raster image. ClearType sub-pixel anti-aliasing is also magnified as a result of this, so if ClearType is active, the magnified text may appear to have unexpected colors at the edges of non-horizontal lines. Some third party magnification software compensates for this effect by applying scaling filters to the enlarged image.

The magnifier also features a lens mode similar to that found in the existing version[which?] of the software. Lens mode is improved, however, as now the magnifying window will follow the cursor around the screen rather than remain in a fixed position. Finally the magnifier is much easier to access by using the Windows key and +/- to control the zoom level without the need to start the application first. Pressing the Win+Esc combination will exit the magnifier.