An agreement with Paulino had been in place for several days, but he had to pass a physical before the contract was finalized.
Paulino, 31, owns a .272 batting average over an eight-year career with the Pirates, Marlins, Mets and Orioles. He was with Baltimore for the first half of last season, but played just 20 games behind Matt Wieters while hitting .254 in 63 at-bats.
The Dominican native finished up last year with the Orioles' Triple-A Norfolk club, hitting .287 with one home run and 15 RBIs in 150 at-bats over 40 games.
The Mariners are looking to bolster their catching position after trading John Jaso to the A's in a three-way deal that brought outfielder Michael Morse from the Nationals.
The 6-foot-3, 250-pound Paulino has excelled against left-handers during his career, posting a .325/.379/.465 line in 604 at-bats against southpaws. His numbers are .247/.298/.631 in 1,263 at-bats against right-handers.
Seattle's only catcher on its current 40-man roster is Jesus Montero, though the club also has extended non-roster invitations to young prospects Mike Zunino and John Hicks as well as Jesus Sucre for the upcoming camp, which opens with pitchers and catchers reporting Feb. 12 in Peoria, Ariz.
General manager Jack Zduriencik said Wednesday he planned to sign two veteran catchers before Spring Training. The club is still interested in adding another free agent, with Kelly Shoppach one possible target.
Shoppach played four years with manager Eric Wedge in Cleveland from 2006-09 and remains unsigned after splitting last year between the Mets and Red Sox.
Other unsigned free-agent catchers include Brian Schneider, Chris Snyder, Matt Treanor, Rod Barajas and Miguel Olivo. Olivo was with the Mariners the past two years, but isn't expected to return.
Yorvit Torrealba, one of the other remaining free agents, agreed to a Minor League deal Thursday with the Rockies. Another potential target, left-handed-hitting George Kottaras, was claimed off waivers by the Royals on Friday. He'd been designated for assignment by the A's on Jan. 16 after Oakland acquired Jaso.
The Mariners split catching duties last season between Olivo, Montero and Jaso, but Wedge said it's possible the club will only go with two this season as Montero matures into more of an everyday option.
"I don't think it'll be that necessary to have three catchers," Wedge said. "It's always a luxury. We definitely needed it last year with the cast of characters we had, but this year I think it might be a little different. But either way, even we only have two catchers, somebody else out there -- and they don't know it yet -- is going to have to figure out how to go back there in the case of emergency. We'll cross that bridge at some point this spring."