"We will use this additional time to evaluate more carefully the impact of such accessions on readiness and lethality," Defense Secretary Jim Mattis wrote in the memo.
In 2016, then-Defense Secretary Ash Carter ended the ban on transgender people being able to serve openly in the military, but he said the process would occur in stages.
Mattis was facing a July 1 deadline for deciding on transgender recruits.
Last week, leaders from the Army, Navy, Air Force and Marine Corps submitted a request
to delay the July 1 deadline to decide whether incoming service members already identifying as transgender could enlist if they have been "stable" in their gender identity for 18 months.
Many Republicans in Congress are opposed to allowing transgender people to serve. An amendment that would have prevented transgender people from doing so was debated during Wednesday's markup of the House's defense authorization bill. The sponsor, Rep. Vicky Hartzler, R-Missouri, said she would wait to see what the Pentagon did before moving forward with her provision.
The American Military Partner Association, which advocates for lesbian, gay, bisexual and transgender rights in the military, said it was disappointed by Mattis' decision.
"This six-month delay is disappointing because it unnecessarily delays the ability of transgender people to be open about their identity when entering the military," the group's president, Ashley Broadway-Mack, said in a statement. "It has been unequivocally proven that allowing qualified transgender people to serve openly strengthens our military and creates a more inclusive and diverse force. The issue has been thoroughly studied, and moving forward with this new recruitment policy is imperative in order for the military to be able to recruit the best talent our nation has to offer."
Stephen Peters, a spokesman for the Human Rights Campaign, argued that allowing transgender people to serve would benefit the military.
"Once this important policy is implemented, it will strengthen our military by allowing qualified and talented transgender people to enlist or commission," said Peters, a Marine Corps veteran who was discharged under "don't ask, don't tell" policy instituted by President Bill Clinton's administration, which barred openly gay people from serving in the military. "Each day that passes without the policy in place restricts the armed forces' ability to recruit the best and the brightest, regardless of gender identity. We are disappointed in this needless delay because the thousands of highly trained and qualified transgender service members openly and proudly serving our nation today have proven that what matters is the ability to accomplish the mission, not their gender identity."