Ah Mau returns to the Warriors, after making his debut for the Kiwi club in 2009.
The 28-year-old spent two seasons with the Cowboys in 2010-11, and has been plying his trade for the last seven years at the Dragons.
“Leeson will be a wonderful addition to our squad from next season,” Warriors head coach Stephen Kearney told warriors.kiwi.
“He’s a big forward who is consistently one of the Dragons’ best week in, week out. He fits the mould of the type of player we’re building our squad around – tough, durable and with a tremendous attitude in all he does on and off the field.
“We’ve always had an interest in him as did a number of clubs, so we’re really excited he was keen to come back home and join the club he started with.”
Ah Mau represented Samoa at the two previous Rugby League World Cups, and made his Test debut for the Kiwis during the recent Denver Test against England.