Kellan Lutz -- Sues Clothing Line ... You Owe Me $1.5 Million

Kellan Lutz says the guy he helped develop a clothing line with turned around and ripped him off to the tune of $1.5 million and now Kellan wants his cash back.

Lutz says in a lawsuit ... he helped develop the "Abbot & Main" clothing company with Daniel Guez in 2013. Lutz says the brand has generated sales in the tens of millions, but he's been short-changed when it comes to royalties.

And get this ... the suit says once Guez got word Kellan was gunning for his cut, Guez started hiding money in accounts that are not in his name -- so it would be hard for Kellan to collect.