The new owner of an 824,772-square-foot office complex in Boston’s Back Bay is a Japanese developer making its first overseas investment.

Mori Trust Co. is scheduled to close today on the acquisition of 10 St. James Ave. and 75 Arlington St. from Liberty Mutual, said Edward Maher, vice chairman at Newmark Grubb Knight Frank in Boston. The property includes the 10-story 75 Arlington St. and 20-story 10 St. James Ave. The price is $673 million, or $816 per square foot.

Liberty Mutual acquired the property for $481 million from Millennium Partners in January 2006. The overall occupancy rate is 93 percent.

The Boston-based insurer in August signed a lease renewal for 324,792 square feet, including the second through 10th floors at 10 St. James Ave., and the sixth floor at 75 Arlington St., according to Suffolk County Registry of Deeds filings.

Mori Trust owns 124 properties in Japan including offices, hotels and residences. In December, Mori Trust announced that it established the subsidiary Mori America LLC to acquire “a stable asset portfolio that can ride out a global economic change.” Mori said it expects to acquire up to $200 billion yen worth of overseas properties, or $1.7 billion.

The Back Bay property attracted 12 bids, including five from outside the U.S., Maher said.

Investors from Australia, Canada, Germany, Norway and Japan have been major players bidding up the price of trophy office buildings in Boston in recent years. With a slowdown in the availability of top-of-the-line properties, there’s been growing demand for smaller acquisitions, said Justin Smith, a managing director at Newmark.

“Foreign capital is still showing up on some of the smaller transactions in Greater Boston and that’s definitely a new trend,” Smith said.