The company is urging customers to avoid a last minute rush so that items arrive in time for Christmas Day.
The last recommended posting date for operational British Forces Post Offices (BFPO) is Monday, November 28.
The deadline for static BFPOs is two weeks later on Monday, December 12.
A spokesman for Royal Mail said: “The partnership between Royal Mail and the military has been in place for nearly 200 years and this important link is even more important at Christmas.
“We would urge people to get their cards and presents in the post as early as possible to ensure that they arrive in time for the celebrations for service men and women to begin.”
The festive period is the busiest time of the year for the UK postal operator which begins planning as early as April.
Royal Mail has operated a dedicated military postal unit since 1882, when the Army Post Office Corps was formed for service during the Egyptian and Sudanese campaigns.
In 1908, the unit was subsumed into the Royal Engineers as the Royal Engineers (Postal Section).
After several more changes in the late 1990s it was re-launched as the BFPO.
Today, the BFPO offers discounted mail delivery to troops stationed around the world.
It also offers free delivery of Forces Air letters (Blueys) sent to personnel on operational duty.