RMS may mean:

• root mean square, a concept in statistics and electronics
• Richard M. Stallman, a computer programmer and founder of the GNU project.
• Royal Mail Ship (or Steamer)
• rate-monotonic scheduling, scheduling technique in operating systems
• Republik Maluku Selatan, a movement in The Netherlands with the goal of establishing a free South Moluccan Republic in Indonesia
• railway mail service
• Record Management Services (OpenVMS)
• records management system
• In power handling (especially music), "watts RMS" is a misnomer for AC average power. See audio power