Mod Sun Gets 'Avril' Tattooed On His Neck, Sign of Serious Relationship

Mod Sun might want to grab a turtleneck the next time he wants to keep his relationship a secret ... 'cause a huge "Avril" tattoo on his neck exposed what everyone's thinking -- he and Avril Lavigne are in a serious relationship.

A hawk-eyed TMZ producer spotted the tat on the back of Mod's neck after the musicians were seen leaving BOA Steakhouse in WeHo. Sources close to the situation tell TMZ ... the tat is 100% the real deal because, as our source put it, there's no way Mod would EVER get a "temp tattoo."

As for when he got the fresh ink -- we're told it was pretty recent. Makes sense, because there's been speculation they're hooking up ever since their collab on the new song "Flames."

Our sources wouldn't confirm a romantic relationship ... but, as they say, the writing's on the Mod -- plus, we're told they're "enjoying each other's company." Feel free to read between the tattoo lines.

As we showed you, Avril and Mod hit up BOA in WeHo Thursday night. No PDA as they exited and greeted some fans, but Mod was pretty gentlemanly as he helped Avril into his whip.

If ya check the neck ... all signs point to Avril having found a new Sk8er Boi.