Marriott Corp. yesterday completed the sale of five hotel properties to Equitable Life Assurance Society of New York for $92 million - a complex deal that will reduce the local restaurant and hotel firm's debts.

Among the properties being taken over by the nation's third-largest insurance firm is the 450-room Twin Bridges here - the first motel-hotel opened by Marriott in 1957 as the company began to expand beyond its initial food services base.

Other hotels sold were in Los Angeles (1,020 rooms), Philadelphia (714), St. Louis (426) and Dallas (476). Marriott will continue to manage the hotels under long-term agreements, part of a corporate strategy to unload real estate and associated debts.

Throughout the hotel industry, many operating companies (such as Marriott) are moving to increase resources for expansion by selling current hotel properties to investors and by finding partners to build and own new units, which Mariott will manage.

The deal with Equitable Life actually involves seven hotels with a total price tage of $119 million. J. W. Mariott Jr., president of the firm, said he hoped to conclude the transaction for a sixth hotel in a month or two and, for the seventh unit, possibly later in 1978.

Mariott declined to identify the other two properties involved: the five hotels involved in yesterday's transaction were first revealed in the company's formal announcement.

"The sale improves our balance sheet significantly while we continue to share substantially in future pofits from the five hotels," Marriott stated.

Initially, the Bethesda-based company will use the $92 million to pay off some debts As of last July 29, the long-term debt of Marriott totaled $342 million Senior debts as a percent of total capital in the firm has declined from 54 percent in 1975 to 45 percent in the last fiscal year and will drop further to about 40 percent with yesterday's sale.

The heavy burden of debt reduced Marriott's flexibility in meeting its twin goals of increasing sales and earnings 15 percent per year.

By selling the hotel properties, "we improve our financial capacity to invest in new hotels and other projects with high returns This will give us operational control of more properties on our existing capital base - and a corresponding earnings increase," Marriott said.

Yesterday's sale increases the number of hotels operated by Marriott under management or lease contracts to 20 with 9,651 rooms. Overall, Mariott operates 38 properties with 17,000 rooms and has announced 12 new hotels scheduled to open in the next two years - most under management contracts, including 7 in the Middle East.

By 1980, the local firm projects 23,000 hotel rooms under management and a few years thereafter the total should increase to about 30,000.

Currently in the final month of fiscal 1978, Marriott is a restaurant, hotel, amusement park and airline catering firm with sales exceeding $1 billion a year. Earnings for the first nine months ended May 5 jumped 26 percent from the previous year and Marriott sayd yesterday that his firm is having "a very strong fourth quarter."

Equitable has assets of more than $24 billion, with real estate holdings exceeding $1.5 billion after yesterday's purchases. "This transaction represents a major expansion in the Equitable's real estate equity investment program," said Robert Schlageter, executive vice president for mortgages and real estate investment at Equitable.