The 28-year-old Lausitzer Füchse forward was presented with his cap by Ice Hockey UK general secretary Andy French in a special ceremony before the game.
Farmer made his GB debut in Ljubljana, Slovenia at the 2012 World Championship Division I Group A and scored his first two goals a year later at the tournament in Budapest.
“It is always an honour to wear the GB shirt, so to reach 50 caps is very special indeed,” said Farmer.
“Playing with this group of guys is brilliant and the spirit in the dressing room is always fantastic.
“We have a great group of guys and we go through the wall for each other, so it has been amazing to share the experiences of the past few years with them.
GB face Estonia at 6pm this evening after Thursday’s opening-day 4-3 victory Romania.