The 30-year-old defender tested positive on Wednesday for high levels of growth hormone HCG, which was found to be caused by the illness.
Frankfurt play Nuremberg in a relegation-promotion play-off tie on Thursday - and Russ is available.
"Despite this tragic diagnosis the player has said he is ready to play," read a Frankfurt statement.
"This is something that has been confirmed by the doctors."
The club have not confirmed the location of the tumour.
Russ has spent almost his entire career with Frankfurt, aside from a spell with Wolfsburg between 2011 and 2013.
Thursday's match is the first of a two-legged tie, with Frankfurt participating after they finished 16th in the Bundesliga.
Frankfurt captain Marco Russ has been diagnosed with a tumour - discovered after he failed a routine drugs test.