In a rematch of their October meeting for the same belt, where an accidental clash of heads left Mathews with a broken nose and resulted in a technical draw, Welshman Rees came back from a sluggish start to overwhelm his game foe.
Rees, a former WBA champion at light-welterweight, will now harbour realistic ambitions of becoming a two-division world champion.
Both fighters came into the contest on the back of impressive wins.
Rees travelled to Paris in March where he systematically dismantled home favourite Anthony Mezaache, while Mathews produced a huge upset to thrillingly rip the British title from Anthony Crolla inside six rounds a month later.
The champion's' customary come-forward style was evident in the opener, although he began to neglect the jab, allowing Mathews to pick him off with a pair of eye-catching combinations and steel the round.
After controlling the distance well over the following three minutes, Mathews found himself in trouble a Rees upped the tempo in the third.
A chopping overhand right from the 32-year-old started a frenzied flurry, the result of which was a cut just above Mathews' right eye as the fighters' heads came together.
Rees soon pinned his man in the corner, unleashing punishing hooks to the body, but the 28-year-old Liverpudlian was unperturbed and ended the session beckoning some central ring trading.
Having landed flush with a pair of right hands to no noticeable effect in five, Mathews' knees momentarily appeared to buckle in the closing second of the round as Rees found his jaw with a shot right hook.
The left to the body proved fruitful for Rees in six and he pro ceded to dominate the middle rounds - the bell coming to Mathews' rescue in eight as a left uppercut and overhand right sent him reeling into the ropes.
In reality the challenger was running on empty and following more relentless pressure, a right flush to the jaw followed by a beautifully timed left from Rees ended the contest.