TOKYO (AP) _ Japan's central bank announced early Monday it will cut its official discount rate to 4.5 percent from 5.0 percent, effective immediately, to restrain an economic slowdown by easing credit.

The decision was made by the Bank of Japan's special policy board, officials said.

Monday's cut was the third this year in the key lending rate, the rate the central bank charges on loans to commercial banks. The bank also cut the discount rate by half a percentage point July 1 and Nov. 14.

The latest measure is expected to help spur Japan's economic growth, sluggish recently.

A Bank of Japan survey conducted last month showed that major manufacturers have revised their forecasts for pretax profits for fiscal 1991, which ends next March 31, sharply downward to show a 12.9 percent decline from the year earlier.

''It is the judgment of the bank that this action will provide sufficient monetary conditions for achieving more balanced non-inflationary growth,'' the central bank said in a statement.

Analysts also linked the discount-rate cut to the forthcoming visit to Japan of President Bush. Some Japanese officials have called for easier credit to bolster domestic demand for imports and thus address the trade imbalance with the United States.

''There might have been strong political pressure on the central bank to lower its key lending rate before President Bush's visit ... because no specific pump-priming measures have emerged yet from the Cabinet of (Prime Minister Kiichi) Miyazawa,'' said Susumu Nozaki, an economist with Tokai Bank.