It restored some credibility to his reputation after serving a 21-month doping ban and then fighting a court battle against the International Olympic Committee before being allowed to run in the 2012 Olympics.
The 27-year-old, who won his previous title in 2009, posted the fastest time of the year of 43.74 seconds to take gold while compatriot Tony McQuay ran a personal best (44.40sec) for second place and Luquelin Santos of the Dominican Republic was third (44.52sec).
Olympic and defending champion Kirani James of Grenada appeared to tighten up in the finishing straight and faded to finish seventh of the eight-man field.
“It just didn’t work out for me today,” said the 20-year-old, who beat Merritt for gold two years ago.
Photos: Amruta Fadnavis at a fashion and lifestyle exhibition
Photos: Sushmita Sen stuns in a black dress with thigh-high slit
Photos: Bhumi Pednekar, Saiyami Kher at a film screening in Mumbai
Dreaded dons! Meet Bollywood's gangster brigade
Photos: The story of Dawood Ibrahim's sister Haseena Parkar