Niners starter Colin Kaepernick opted out of his contract this offseason, leaving San Francisco without a quarterback under contract. Hoyer agreed to his deal -- believed to be for two years and around $6 million per season -- with the understanding that he will get a chance to start.
In San Francisco, Hoyer will be reunited with Kyle Shanahan. The new 49ers head coach was the Cleveland Browns' offensive coordinator in 2014, when Hoyer started for the team. Hoyer started strong that season but struggled over his final five games.
Hoyer, 31, performed well in five starts for the Chicago Bears last year, but his season ended when he broke his left arm at Green Bay in Week 7. He threw for 1,445 yards, six touchdowns and no interceptions.
An eight-year NFL veteran, Hoyer signed with Chicago last offseason partly because of the strong relationship he enjoyed with Bears offensive coordinator Dowell Loggains, who previously served as Hoyer's quarterbacks coach in Cleveland.
Juszczyk, who turns 26 on April 23, has spent his entire career with the Baltimore Ravens. He reached his first Pro Bowl last season after being rated the NFL's highest-rated fullback by Pro Football Focus.
He led all fullbacks in receptions the past two seasons, catching a total of 78 passes for 587 yards and four touchdowns.
Hoyer started 16 games for the Browns from 2013-14, and he made nine regular-season starts for the Houston Texans in 2015, passing for 2,606 yards, 19 touchdowns and seven interceptions. In his only playoff start, however, Hoyer threw a career-high four interceptions in a loss to Kansas City, which ultimately led to his release.
The Niners also are expected to sign wide receiver Pierre Garcon, sources told ESPN's Chris Mortensen.
ESPN's Adam Caplan, Adam Schefter, Jeff Dickerson and Jamison Hensley contributed to this report.