The decision to join talks on the Trans-Pacific Partnership (TPP) launches the "third arrow" in Abe's policy triad following the fiscal pump priming and hyper-easy monetary measures he has pushed since returning to office in December after his Liberal Democratic Party's (LDP) big election win.
"Emerging countries in Asia are shifting to an open economy one after another. If Japan alone remains an inward-looking economy, there would be no chance for growth," Abe told a news conference.
"This is our last chance. If we miss this opportunity, Japan will be left behind."
The government estimates that joining the TPP will boost Japan's gross domestic product by 3.2 trillion yen ($33.3 billion), or 0.66 percentage points, offsetting the negative impact on agriculture by boosting exports in other sectors and domestic private consumption.
"Abenomics" has been playing to rave reviews in the Tokyo stock market and with voters, around 70 percent of whom support the prime minister.
Business executives and economists say the real test, though, will be whether Abe buckles down to more controversial reforms such as deregulation, which can hurt vested interests.
"TPP could be a trigger for Japan to implement deregulation in various sectors by using external pressure," said Hideo Kumamo, chief economist at Dai-ichi Life Research Institute.
"It is being seen as a way to stimulate the economy by making the nation more competitive."
The United States and 10 other countries are pushing for a deal by the end of the year and possibly as soon as an Asia-Pacific leaders summit in Bali in October.