A 26-year-old Georgetown University graduate pleaded guilty today to federal charges of trying to extort $2 million from Washington shipping broker Harry J. Smith Jr.

Charles B. Dwyer of Methuen, Mass., was given a two-year suspended sentence, placed on two years' probation and fined $500.

Dwyer and Martin Donohoe, a college classmate and former Smith employe, mailed documents about Smith's business transactions to The Washington Post early this year as part of a plot to extort the money from Smith, according to the charges.

The Post printed an article about Smith's dealings with the Jamaican government on Feb. 12. It said Smith's company, St. John International, directed Jamaica's grain shipping business to a firm in which he once had a financial interest.

That firm, Agrobulk Shipping Corp., hired other companies to ship Jamaica's grain imports and made $900,000 profit in 1975, according to documents cited in the report. This was because the actual cost of the shipping was less than Jamaica paid Agrobulk.

The Post article also said Smith directed unauthorized commissions to the Swiss bank account of another firm which then made payments to two Jamaican trade officials. Those officials have been under investigation for taking kickbacks.

Seymour Glanzer, Smith's attorney, said in February that The Post Was "unwittingly used" in the extortion plot. But he did not dispute the thrust of the report.

Though the article said Smith apparently broke no U.S. laws in the transactions, Jamaican officials were upset by the revelations. They have since replaced Smith as a shipping broker.

Dwyer was arrested Feb. 16 in Rye, N.H., after Smith told the FBI about allegedly extortionate phone calls and the FBI traced them to the home of Dwyer's brother in Rye.

Authorities said Dwyer began cooperating immediately after his arrest.An investigation is continuing, but not charges have been placed against Donohoe. He left the country in February and is believed to be in France.

William Shaheen, U.S. attorney for New Hampshire, said today that he felt Dwyer was used by Donohoe, who authorities view as the mastermind of the plot.