Niemann will earn $2.75 million instead of his request for $3.2 million, arbitrators James Oldham, Howard Edelman and Mark Irvings decided after hearing arguments a day earlier.
Tampa Bay improved to 6-0 in arbitration.
Niemann was 11-7 with a 4.06 ERA in 23 starts last season, when he didn't pitch between May 4 and June 20 because of a strained back. He made $903,000.
In the first decision of the year, Washington pitcher John Lannan lost Thursday and will make $5 million rather than his request for $5.7 million.
Miami Marlins pitcher Anibal Sanchez asked a panel for a raise from $3.7 million to $8 million Friday, with the team arguing for $6.9 million. Sanchez, who is eligible for free agency after this season, was 8-9 with a 3.67 ERA in 32 starts last year, when he struck out 202 in 196 1-3 innings.
The three cases argued already matched last year's total, which tied a record low. Nineteen players remain scheduled for hearings, which run through Feb. 17.
Two right-handed pitchers settled on one-year contracts just before the scheduled start of hearings Friday. The Chicago Cubs and Matt Garza agreed at $9.5 million, and the Milwaukee Brewers and Shaun Marcum agreed to $7,725,000.