STDWeb performs photometry and transient detection in astronomical images through a web-based interface. It accepts FITS images and provides interactive masking, detects sources, performs astrometric calibration, and calibrates photometry with supported catalogs and filters. The software subtracts templates using either automatic or user-provided images and performs forced photometry on targets in original or difference images. STDWeb includes experimental transient detection with basic artifact rejection. The code relies on STDPipe (ascl:2112.006) and uses several external tools, including SExtractor (ascl:1010.064), HOTPANTS (ascl:1504.004), and Astrometry.net (ascl:1208.001). STDWeb runs as a web service with a back-end task queue suitable for deployment in observatory or archive environments.