The service, held at San Francisco de Asis church, saw Hopper's family, friends and neighbours gather to remember the actor, reports The AP.
Nicholson, who described the star as an "all-around guy", said: "It was a very singular relationship I had with him, like nobody else. We were soulmates in a way. I really miss him."
Hopper's son read from Walt Whitman's Leaves of Grass, while brothers Marvin and Davis also spoke during the service.
Hopper died on Saturday after a long battle with prostate cancer. He was 74.