The 49-year-old American was awarded the fight 116-111 by two judges but Gustavo Padilla controversially had it 114-113 for the 30-year-old Shumenov, who was floored by a big right hand in the 11th round.
Hopkins improved his record to 55-6-2 with 32 knockouts after dominating his much younger opponent, who suffered his second career defeat and slipped to 14-2-0.
After a cagey opening in Washington, Hopkins took control of the contest and tellingly dictated the pace after landing a number of telling right hands through some sluggish defence by Shumenov, who fought in the 2004 Olympics.
The taller Kazak stepped up his attack in the eighth, taking a couple of rounds after landing some strong lefts before being beaten to the punch by Hopkins in the 11th and dropping to one knee.
Hopkins also extended his record as the oldest winner of a world title and his confident display suggested he had no plans to end a professional career which began in defeat in 1988.