The comment in an interview with Japan's main financial newspaper, the Nikkei, came ahead of a trip to Japan this weekend and represents the clearest U.S. support yet for Tokyo's effort to bolster its military as it faces off against a more assertive China.
"We welcome Japan's efforts to play a more proactive role in the alliance, including by re-examining the interpretation of its constitution relating to the right of collective self-defense," Hagel said in a written response to the Nikkei.
Hagel visits China, suspicious of Japan's military intentions and where memories of Japan's past militarism run deep, after Tokyo.
Japan has been locked into a security pact for more than half a century that commits the United States to defend Japan should it come under attack. Japan's Self-Defense Forces, however, are not allowed to aid U.S. ships or other military units that are fired upon.
Adopting a policy of collective self-defense is a cornerstone of hawkish Prime Minister Shinzo Abe's policy to bolster the military. His critics say doing so would weaken constraint on using the defense force.
In its postwar pacifist constitution, written by the United States, Japan renounced the right to wage war.
Clear backing for collective self-defense by the United States, which is discussing with Japan ways to update their security pact, may help assuage concern in Tokyo that Washington will favor China as its economic power grows.
Hagel's visit to Japan comes ahead of a planned trip to Tokyo by President Barack Obama later this month.
Japan eased its weapons export restrictions on Tuesday in the first major overhaul of arms transfer policy in nearly half a century in a move that alarmed China.
It came as Sino-Japanese ties have been chilled due to a territorial dispute over a group of East China Sea islets and Abe's visit in December to Tokyo's Yasukuni Shrine, seen by critics as a symbol of Japan's wartime aggression.