By TMZ

Former light heavyweight champion Sergey Kovalev has been charged with felony assault for allegedly punching a woman in the face back in June -- but he claims he's not guilty ... TMZ Sports has learned.

Sources connected to the case tell us ... the woman is claiming Kovalev had hit on her at a party earlier in the evening but rejected his advances. She claims Sergey and a friend followed her back to her cabin and that's where the alleged attack occurred.

We're told the woman claims she suffered major injuries including a severely broken nose, a concussion and a displaced disk in her neck.

Cops tell TMZ Sports ... they began searching for Kovalev immediately and arrested him a short time later.

He was charged with assault by means of force likely to cause great bodily injury -- which carries a maximum sentence of 4 years in prison.

Kovalev appeared in court and entered a not guilty plea.

What's interesting ... Kovalev fought Eleider Álvarez in August -- just 2 months after the arrest -- and lost in a shocking upset.

Kovalev is due back in court in March. He also has a rematch with Alvarez lined up for Feb. 2.

We've reached out to Kovalev for comment -- so far, no word back.