For each weigh-in that Lacy reaches his target number, he can make $55,000.
The numbers are 255 pounds in May; 250 pounds in June and August; and 245 pounds in September, October, November and December.
Lacy weighed 231 pounds coming out of Alabama in 2013. The Milwaukee Journal Sentinel reported that he was up to 267 pounds during one of his free-agent visits this offseason.
Pete Carroll said this week that Lacy can be most effective playing in the 240s.
"I want him big. I want him big and tough and strong," the Seahawks coach said on The John Clayton Show on 710 ESPN Seattle. "But I want him in the best shape so that he can run at his best and be durable and handle the load -- 235, that's at the time he came out [of the draft], he was there.
"But he's developed. Into his [240s], he's really dangerous."
Carroll added that Lacy has been somewhat limited in his conditioning work, because he is still rehabbing from ankle surgery.
The Seahawks and Lacy agreed to a one-year contract that includes $2.865 million guaranteed. Lacy can earn an additional $2.685 million through incentives, including the weight clause.
A second-round draft pick by the Green Bay Packers in 2013, Lacy posted back-to-back 1,100-yard seasons to start his pro career and was the NFL's Offensive Rookie of the Year in 2013.
But his weight became an issue in 2015, when Green Bay coach Mike McCarthy said Lacy needed to come back in better shape after rushing for just 758 yards.
He worked out with P90X founder Tony Horton last offseason and was effective in five games, averaging 5.1 yards on 71 carries before suffering the ankle injury.