Smith, a 20-1 on favourite at the first bell, had some anxious moments and was cut badly around the right eye, but took the decision 117-111, 116-112, 116-112 on the judges' scorecards.
Ryder never took a backward step and the decision was met with some derision among the crowd.
"I didn't underestimate John in training," Smith told Sky Sports Main Event.
"I knew he was a tough fighter and it was going to be that type of fight, that he was going to get really close and stop me from what I'm good at.
"It wasn't the best performance but I thought I won.
"He worked hard at the end, but I was due a bad one. It's the first time I've been cut in a fight and I'll be a better boxer because of it."
Ryder has been a notoriously slow starter during his career, but the 31-year-old Londoner began in confident fashion and possibly shaded the opening two rounds.
Smith's usual snap in his punching was lacking and he struggled to make his significant reach advantage count against the compact Ryder.
But Smith launched a flurry of punches at the end of the fifth round, and noticeably had more success when he increased the distance between the two men.
Ryder refused to be fazed, though, and landed a heavy left in the seventh that had the favourite covering up on the ropes.
There was a nasty clash of heads in the eighth and Smith was clearly upset by another collision in the ninth as he protested to the referee.
Ryder had Smith wobbling in the 11th round with a left hook as he sought to finish the fight.
But Smith held on and was relieved to hear the final bell and to keep hold of his world belt.