The charity adopted the Portishead and Bristol lifeboat from Portishead Lifeboat Trust in a ceremony earlier.
A £1.9m purpose-built centre has been constructed on the site of a former Masonic lodge next to the marina.
Bob Crane, RNLI lifeboat operations manager for Portishead, said it was a "momentous day".
"While it has taken 12 months to build the new lifeboat station, it has been 20 years since the Portishead Lifeboat Trust was set up by a group of independent volunteers who recognised the need for a sea rescue service to cover this stretch of the Severn Estuary.
"We have come a very long way since then and along the way launched on 381 operational shouts, saved 14 lives and assisted a further 483 people."
An anonymous donor gave more than £500,000 to help fund the project last year.
A new lifeboat station in North Somerset has become operational after it was formally adopted by the Royal National Lifeboat Institution (RNLI).