The ambrotype process (from Greek ambrotos, "immortal") is a photographic process invented in the mid-1850s by Frederick Scott Archer. The process creates a glass negative, which appears as a positive when placed against a black background.

One side of a very clean glass plate is covered with a thin layer of collodion, then dipped in a silver nitrate solution. The plate is exposed to the subject while still wet. (Exposure times vary from five to sixty seconds or more depending on the amount of available light.) The plate is then developed and fixed. The resulting negative, when viewed by reflected light against a black background, appears to be a positive image: the clear areas look black, and the exposed, opaque areas appear light. This effect is achieved by coating one side of the glass negative with black varnish. Either the emulsion side or the blank side can be covered with the varnish: when the blank side is blackened, the thickness of the glass adds a sense of depth to the image. In either case, another plate of glass is put over the fragile emulsion side to protect it, and the whole is mounted in a metal frame and kept in a protective case.

The ambrotype was much less expensive to produce than the daguerreotype, and it lacked the daguerreotype's shiny metallic surface, which some found unappealing. By the late 1850s, the ambrotype was overtaking the daguerreotype in popularity; by the mid-1860s, the ambrotype itself was supplanted by the tintype and other processes.

Ambrotypes were often hand-tinted. Untinted ambrotypes are grayish-white and have less contrast and brilliance than daguerreotypes.