Every year more than 300,000 people living in such places as Poland, India, Hong Kong and the mountain villages of Greece and Italy receive $600 million in checks from the U.S. Social Security system.

Although few people are aware of it, under U.S. Social Security law, anyone who has earned the right to Social Security benefits is entitled to receive them, even if he or she has decided to spend his retirement years abroad.

That applies to U.S. citizens who retire abroad as well as to non-citizens who worked in the United States for many years and then went back to their native countries.

And it also applies to dependent widows and other survivors of these beneficiaries, even if the survivor has never lived in the United States at all.

The $600 million is a small fraction of the huge $90 billion old-age and survivor disability benefit program, but it is a substantial amount.

Mexico, with 47,142 beneficiaries at the end of 1977, was the leading recipient country, but Canada was a close second (45,345) and Italy was third (42,367). The Philippines (34,642) was fourth.

Other leaders were West Germany (17,341), Greece (16,878), Britain (12,717), Ireland (6,798), Spain (6,631) and Israel (6,486), where American Jews form a retirement colony.

But there were small numbers virtually all over the globe-New Zealand, Cyprus, Thailand, Turkey, Latin American, Poland.

The large number of Italian beneficiaries, according to one official, is due in part to the wave of pre-World War I immigration of Italians to the United States. In many cases, the husband came without his wife, lived here for many years and earned Social Security benefits, without getting divorced from his wife back home. Today, many of these elderly widows, who hadn't seen their husbands for years and who had never left Italy, are receiving widow's benefits.

Overseas recipients get only cash payments-not Medicare, unless they come back. The check is in U.S. dollars.

In some countries no checks can be paid. That is because of 1940 Treasury regulations that bar payment on checks in countries where there isn't reasonable assurance the individual would receive it and be able to negotiate it at full value. Today these countries include Albania, Cuba, East Germany, North Korea, Vietnam and the People's Republic of China. The Soviet Union is no longer on the ban list, but benefits aren't paid to people there because the Soviet government has refused to cooperate in administratering the checks (checking addresses, death lists and so forth).

A Social Security official said overseas recipients once a year must send in a form certifying, in effect, that they are alive and still eligible and not earning over the allowable limit for those receiving benefits.

There was a time, when American currency was extremely valuable overseas, when some U.S. retirees felt they could beat the system by retiring abroad, where the dollar bought a lot.

No longer. A Social Security official said the number of beneficiaries choosing retirement abroad may shrivel because the dollar has lost purchasing power in relation to such currencies as the Swiss franc, West German mark and Japanese yen. Now it buys less.

"Americans who thought they could go abroad and live comfortably on Soccial Security a few years ago now find their money buys half as much," he said.

"You may be okay in a mountain village in Greece, but not in Zurich."