Japan's $1.1 trillion retirement fund posted its best annual return on record, buoyed by gains in stocks and a weaker yen.

The Government Pension Investment Fund delivered a 12 percent return in the year that ended March 31, the most since its 2001 inception, it said in a statement. Assets under management swelled to ¥137.5 trillion ($1.1 trillion), also a record, as domestic stocks gained 30 percent. The yen's 14 percent decline against the greenback helped overseas equities provide a 22 percent return, while local bonds made 2.8 percent.

GPIF's asset mix is closing in on the targets outlined in a radical overhaul in October, when the fund vowed to cut its Japanese bond allocation to 35 percent and more than doubled its goals for equities. Its performance underscores the argument for increasing risk assets as it seeks to meet the pension needs of the world's oldest population.