"After consulting with the service chiefs and secretaries, I have determined that it is necessary to defer the start of accessions for six months," Mattis wrote in a Friday memo obtained by The Associated Press.
"We will use this additional time to evaluate more carefully the impact of such accessions on readiness and lethality," Mattis stated, cited by Military.com.
The delay in allowing the recruitment of openly transgender individuals will not affect those who are already serving in the US military, according to the memo.
Mattis has stated that the review must be completed by December 1, noting that there must be one standard to ascertain whether a transgender individual is fit to serve in the US military.
The additional time will allow for "the benefit of the views of the military leadership and of the senior civilian officials who are now arriving in the department," he wrote.
The memo from the US Secretary of Defense saw varying responses, including a Marine veteran who asserted that, "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."
Another noted that the delay announced by Mattis "makes no sense because, as predicted by all of the research, transgender military service has been a success."
US service members who are transgender have been able to serve openly since last year, when previous Defense Secretary Ashton Carter ended a ban.
Beginning October 1, 2016, transgender service members have been able to receive medical care and formally change their gender identification within the Pentagon database.
A recent study noted between 2,500-7,000 transgender service members in active duty in the US, and an additional 1,500-4,000 in the reserve
