International Business Machines Corp. and a Boston-based developer have announced the formation of a joint-venture partnership to develop a $125 million office park in Montgomery County. It will be IBM's second joint venture in the Washington area.

The plan calls for phased-in construction of seven office buildings, totaling 1 million square feet of space on 35 acres at Shady Grove Executive Center at I-270 at Shady Grove Road in Gaithersburg, according to the development company, Spaulding & Slye.

The new office park will be second only to the federal government in the amount of office space occupied in the Washington area. The venture is part of a recent move by IBM to build offices and rent some of them, company officials said.

IBM occupies 2.274 million square feet of space in the metropolitan area, mostly in Northern Virginia and suburban Maryland, officials said.

"We feel Shady Grove Executive Center is one of the premier office complexes in the Washington area, and we are pleased to be part of the project," said Arthur J. Hedge, vice president of IBM's real estate construction division. "In addition, we welcome the opportunity to enter into another partnership with Spaulding & Slye."

The project's office space includes an existing 109,000-square-foot building that is not part of the joint venture but that IBM occupies entirely.

Shady Grove Executive Center 2, the partnership's first jointly owned building, also will be occupied by IBM later this year. That four-story building contains 102,000 square feet of space.

A second jointly owned building, Shady Grove Executive Center 3 -- a five-story, 120,000-square-foot complex -- is scheduled for completion in December.

Spaulding & Slye will act as managing general partner and leasing agent during the entire project, which should be completed by 1990, said Randall W. Byrnes, the company's senior vice president and regional manager.

Byrnes said that the firm is "close to signing a leasing agreement" with a District-based company for Shady Grove Executive Center 3, but declined to name the prospective tenant.

Neither IBM nor Spaulding & Slye would divulge the size of their ownership shares in the new venture.

This is the sixth joint venture IBM has announced since last October. The corporation's four other joint ventures are in Philadelphia, Dallas, Atlanta and New York City. When completed, those projects will contain a total of 3.55 million square feet of office space, of which IBM will occupy 1.63 million square feet, company officials said.

IBM's first Washington-area joint venture with Spaulding & Slye, Rockledge Centre, was announced last January. The venture consists of two office buildings, each with 250,000 square feet of space, in a Montgomery office park near I-270 and the Capital Beltway, Byrnes said.

IBM will occupy the entire building that is scheduled for completion late this summer. The second, expected to be ready for occupancy in January 1986, will be leased, Byrnes said.

Shady Grove Executive Center initially was a joint venture of Spaulding & Slye and Copley Real Estate Advisors, an affiliate of New England Mutual Life Insurance Co. The new venture involves all three companies.