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.
Arbitrators James Oldham, Howard Edelman and Mark Irvings heard arguments Thursday. A decision is expected on Friday.
Tampa Bay is 5-0 in arbitration.
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.