Having claimed four race wins during the regatta, the Australian pair's fourth-place finish in the medal showdown on Saturday was enough for them to claim the gold ahead of Sime Fantela and Igor Marenic of Croatia, with Greece's Panagiotis Mantis and Pavlos Kagialis claiming bronze.
The win sees Belcher defend the title he claimed in 2011 and become the first man to seal five successive 470 world championship events in a row.
"It is really cool. Amazing," said Belcher, whose triumph came on his 32nd birthday.
"It has been so up and down here. We are really happy with how we have performed and it is nice to come away on top," he added on the event's official website.
In the women's 470, European champions Lara Vadlau and Jolanta Ogar beat the New Zealand boat of Jo Aleh and Polly Powrie to the title in the Spanish port of Santander by winning the medal race.
Aleh and Powrie could only manage sixth and were forced to settle for silver, with Great Britain's Hannah Mills and Saskia Clark taking bronze.
The penultimate day of racing was hit by cancellations as the final two fleet races of the Finn class were cancelled due to poor wind conditions, leaving Britain's Giles Scott in command with a 20-point lead.
New Zealand's Peter Burling and Blair Tuke are a healthy 36 points clear going into their medal race on Sunday after finishing fifth in the only 49er class race of the day.
The situation is much tighter in the 49erFX, where Ida Marie Baas Nielsen and Marie Thusgaard Olsen of Denmark lead Brazil's Martine Soffiatti Grael and Kahena Kunze by two points after very contrasting days.
Nielsen and Olsen came fifth, third and ninth in the class' three races, while Grael and Kunze added a victory in the final pre-medal contest in addition to two further top-three finishes.
In the Nacra 17, French pair Billy Besson and Marie Riou cemented their lead by coming fifth in their sole race of the day, with closest rivals Vittorio Bissaro and Silvia Sicouri only good enough for 11th.