His comments came after he met last week with the top Hamas leaders in Syria.
Carter also said Hamas won't undermine Palestinian President Mahmoud Abbas' efforts to reach a peace deal with Israel. He said Hamas is ready to accept a Palestinian state made of the West Bank and Gaza.
Carter made the comments during a speech in Jerusalem on Monday.
The former U.S. president also said it's a "problem" that Israel and the U.S. refuse to meet with Hamas.
"The problem is not that I met with with Hamas in Syria," he said. "The problem is that Israel and the United States refuse to meet with someone who must be involved."
Carter also said Israeli-Palestinian peacemaking has "regressed" since a U.S.-hosted Mideast conference in Annapolis, Maryland, in November.
Speaking about the possibility of renewed peace talks between Israel and Syria, he said Syria wants the U.S. to play a "strong role" in bringing to two sides together.
He also said Hamas has promised to let captured Israeli soldier Gilad Shalit send a letter to his parents.
The State Department twice advised Carter against meeting Hamas leaders before he left on his Mideast trip early last week. Secretary of State Condoleezza Rice criticized Carter's plans to meet Hamas chief Khaled Mashaal in Syria. It was the first public contact in two years between a prominent American figure and Hamas' leadership
Several members of Congress also urged Carter not to meet Mashaal, saying it would confer legitimacy on the group behind some 250 suicide bombings against Israelis that have killed numerous civilians.
But Carter, who brokered the 1978 Israeli-Egyptian peace and won the Nobel Peace Prize in 2002, defended what he called his personal peace mission, saying Hamas must be engaged in order to achieve Israeli-Palestinian peace.
The United States designated Hamas a terrorist organization in January 1995, which made it a violation to conduct any financial or business transaction with the group.
Shortly after Hamas claimed responsibility for an August 19, 2003 suicide bombing in Jerusalem that killed 20 people including four U.S. citizens, the Treasury Department's Office of Foreign Asset Control named a number of Hamas leaders as "specially designated global terrorists." They included Mashaal and Abu Marzouk and the designation made it illegal to conduct any transactions with them.
Israel also brands Hamas a terrorist organization and has accused Mashaal of masterminding the kidnapping of Shalit near Gaza two years ago. Israel has also blamed Mashaal and the group's Damascus-based leadership of directing suicide bombings such as the September 2004 attacks that killed 16 Israelis in the southern city of Beersheba.
Israel tried to kill Mashaal in 1997, when agents sprayed him with poison on a street in Amman. Jordan's late King Hussein, who had signed peace with Israel in 1994, forced Israel to send the antidote that saved his life.
Afterward, Jordan expelled Mashaal to Qatar as the kingdom's ties with Hamas deteriorated, and he moved to Damascus in 1999.