WASHINGTON — A longtime high-ranking official at the State Department was sentenced to just over a year in jail yesterday for keeping thousands of classified documents at his home and for lying about his personal relationship with a younger Taiwanese diplomat.

In late 2005, the former deputy chief of the State Department’s East Asian and Pacific Affairs Bureau, Donald Keyser, pleaded guilty to three felony charges stemming from an investigation into his contacts with the Taiwanese agent, Isabelle Cheng.

Judge Thomas Ellis of Alexandria, Va., imposed the sentence after a lengthy hearing yesterday. According to court records, the judge ordered Keyser to spend a year and a day in jail and two years on supervised release, and to pay a fine of $25,000.

Keyser had asked not to be sent to jail. He submitted a sheaf of letters from former colleagues praising him for his selfless devotion to America during more than three decades in the Foreign Service.

“Mr. Keyser had an absolute obligation to safeguard the classified information entrusted to him and utterly failed to do so. His sentence of imprisonment is a warning to others in positions of public trust,” the U.S. attorney overseeing the case, Chuck Rosenberg, said in a written statement.

Keyser’s sentencing was delayed repeatedly after prosecutors accused him of failing to keep his promise to cooperate in debriefings with the FBI. They also alleged that polygraph tests showed that Keyser was not telling the government the full truth about what transpired.

Keyser’s attorney, Robert Litt, said the government wanted Keyser to admit to spying for Taiwan, when, in fact, he had not done so.

As the two sides wrangled, prosecutors disclosed that FBI agents had observed what appeared to be intimate encounters in an automobile between Keyser, now 63, and Ms. Cheng, who was in her 30s. The government also disclosed personal e-mails between the pair.

“I’m glad the background helped. By now you know that, as we say, ‘Your wish is my command,'” Keyser wrote to Ms. Cheng in 2003.

Keyser admitted to an improperly intimate relationship with his Taiwanese colleague. He also acknowledged that he hid the close ties and a secret trip to Taiwan to meet Ms. Cheng, but he adamantly denied giving her any information that was not authorized for release to Taiwanese officials.

Ms. Cheng, who was openly affiliated with the unofficial Taiwanese post in Washington, has not been charged in the case.

As part of the plea deal, Keyser also admitted that he had more than 3,000 classified documents at his home, some of them marked “top secret.” He said he didn’t realize that the documents were at his house as a result of a series of office moves, but he acknowledged that he failed to act when his wife, who is a top intelligence official, told him some of the records were classified.

When the prosecution and defense were at loggerheads last summer, a prosecutor spoke in court of bringing tougher, espionage-related charges against Keyser. No additional charges were filed, and the prosecution eventually dropped its request to back out of the plea deal.