Martin, 33, will join the Lions’ city competitions to a two-year contract with an option of a further 12-weeks on July 1st.
The son of Irons legend and FA Cup winner Alvin Martin, the former MK Dons shot stopper established himself as Neil Harris’s number one at the second half of the year as they narrowly avoided relegation.
“Signing for West Ham United is a dream, both for me and my family,” Martin advised the club’s official site.
“I have grown up around West Ham and also to come into my boyhood club is a fantasy.
“I arrived into games as a child among the fans from the stands at the Boleyn Ground.
“I enjoyed seeing my father play, and I have continued coming to matches during my career”
Martin created 15 looks for Millwall throughout his two-year remain at The Den, 11 of that came this season.
West Ham declared the signing only days following the coming of Spaniard Roberto, a goalkeeper from Espanyol, and the two are expected to give opposition to Lukasz Fabianski for the number one jersey.