OMEMO is an extension to the Extensible Messaging and Presence Protocol (XMPP) for multi-client end-to-end encryption developed by Andreas Straub. According to Straub, OMEMO uses the Double Ratchet Algorithm "to provide multi-end to multi-end encryption, allowing messages to be synchronized securely across multiple clients, even if some of them are offline". The name "OMEMO" is a recursive acronym for "OMEMO Multi-End Message and Object Encryption". It is an open standard based on the Double Ratchet Algorithm and the Personal Eventing Protocol (PEP, XEP-0163). OMEMO offers future and forward secrecy and deniability with message synchronization and offline delivery.
In comparison with OTR, the OMEMO protocol offers many-to-many encrypted chat, offline messages queuing, forward secrecy, file transfer, verifiability and deniability at the cost of slightly larger message size overhead.
The protocol was developed and first implemented by Andreas Straub as a Google Summer of Code project in 2015. The project's goal was to implement a double-ratchet-based multi-end to multi-end encryption scheme into an Android XMPP-based instant messaging client called Conversations. It was introduced in Conversations and submitted to the XMPP Standards Foundation (XSF) as a proposed XMPP Extension Protocol (XEP) in the autumn of 2015 and got accepted as XEP-0384 in December 2016.
In June 2016, the non-profit computer security consultancy firm Radically Open Security published an analysis of the OMEMO protocol.
Selected clients supporting OMEMO (full list of clients also exists):
- BeagleIM (macOS)
- ChatSecure (iOS)
- Conversations (Android)
- Converse.js (Browser-based)
- Dino (Linux, macOS)
- Gajim via official plugin (Linux, Windows, BSD)
- Monal (iOS)
- Psi via official plugin (Linux, Windows, macOS)
- Psi+ via official plugin (Linux, Windows, macOS, Haiku, FreeBSD)
- libpurple clients such as Pidgin or Finch via experimental plugin 
- Adium via an Xtra based on the libpurple plugin
- Profanity via experimental plugin (BSD, Linux, macOS, Windows)
- SiskinIM (iOS)
- Smack supports OMEMO using the two modules smack-omemo and smack-omemo-signal
- XMPPFramework (macOS, iOS, tvOS)  supports OMEMO via the OMEMOModule extension  when used in conjunction with the SignalProtocol-ObjC library.
- "[Standards] NEW: XEP-0384 (OMEMO Encryption)". 2016-12-07. Retrieved 2016-12-23.
- Daniel Gultsch. "OMEMO Multi-End Message and Object Encryption". Retrieved 2015-11-23.
- "OMEMO Multi-End Message and Object Encryption". conversations.im. Retrieved 2019-01-22.
- "ChatSecure iOS v3.2.3 - XMPP Push". 2016-07-25. Retrieved 2016-09-07.
- "ChatSecure v4.0 - OMEMO and Signal Protocol". 2017-01-17. Retrieved 2017-02-07.
- Bahtiar Gadimov (2015-12-26). "Initial OMEMO commit". dev.gajim.org. Retrieved 2017-09-08.
- OMEMO: Cryptographic Analysis Report. June 2016
- "Are we OMEMO yet?". Are we OMEMO yet?. Retrieved 2019-01-22.
- "BeagleIM and SiskinIM just got OMEMO support". tigase.net. Retrieved 2019-06-07.
- "ChatSecure v4.0 - OMEMO and Signal Protocol". chatsecure.org. Retrieved 2017-01-17.
- "XEP-0384: OMEMO Encryption · Issue #497 · conversejs/converse.js". GitHub. Retrieved 2019-11-20.
- "Dino - Modern Jabber/XMPP Client using GTK+/Vala". dino.im. Retrieved 2017-11-06.
- Bahtiar Gadimov; et al. "Omemogajimplugin · Wiki · gajim / gajim-plugins · GitLab". Retrieved 2016-12-04.
- Vyacheslav Karpukhin. "OMEMO for Psi · GitHub". Retrieved 2018-03-04.
- Vyacheslav Karpukhin. "Psi+ snapshots". Retrieved 2018-03-04.
- Richard Bayerle. "lurch - OMEMO for libpurple". Retrieved 2017-02-12.
- Olivier Mehani. "Lurch4Adium - OMEMO Xtra for Adium". Retrieved 2017-06-08.
- René Calles. "profanity-omemo-plugin: A Python plugin to use (axolotl / Signal Protocol) encryption for the profanity XMPP messenger". Retrieved 2017-01-10.
- Schaub, Paul. "Ignite Realtime Blog: Smack v4.2 Introduces OME... | Ignite Realtime". community.igniterealtime.org. Retrieved 2017-07-11.
- XEP-0384: OMEMO Encryption (Experimental)
- Python library for implementing OMEMO in other clients
- OMEMO protocol implementation in C
- OMEMO Top - OMEMO support toplist in instant message clients