Judy Gross said that in the letter, which Castro read but did not respond to, she pleaded with him to free her husband Alan because their daughter has been diagnosed with breast cancer and he is needed at home.
Alan Gross, 61, who worked for a Washington-area company contracted under a U.S. Agency for International Development program to promote democracy in Cuba, was arrested at the Havana airport on December 3 and has been held on suspicion of espionage and subversion.
In an interview this weekend, his wife denied he was a spy and said he went to Cuba five times last year to help Havana's Jewish community gain Internet access to Jews worldwide.
Cuban officials say Gross committed "serious crimes" by giving restricted satellite communications equipment to local dissidents, but no legal charges have been filed.
His detention has stalled efforts by Washington to improve ties with the communist-led island.
Judy Gross criticized the White House for not doing enough to seek release of her husband, whom she called a "pawn" caught up in a decades-old ideological feud between the United States and Cuba. She said she has heard nothing from President Barack Obama.
The White House said on Sunday it shared her "concern and frustration with the continued unwarranted detention of her husband."
"Administration officials have repeatedly made clear to Cuban authorities that Alan Gross should be released immediately to be able to rejoin his wife and family -- and we will continue to do so," National Security Council spokesman Mike Hammer said in a statement.
In an August 4 letter to Castro, she wrote, "I recognize today that the Cuban government may not like the type of work that Alan was doing in Cuba."
"But I want you to know that Alan loves the people of Cuba, and he only wanted to help them. He never intended them, or your government, any harm," she said.
"To the extent his work may have offended you or your government, he and I are genuinely remorseful," she wrote.
She told Castro her family needed Gross home since his 26-year-old daughter, whose name she asked not be used, was diagnosed with cancer and is undergoing chemotherapy.
The only response came at a meeting this month with Jorge Alberto Bolanos, head of the Cuban Interests Section in Washington, who offered mother and daughter visas to visit Gross in Cuba. He said President Castro had read her letter.
