An overlay journal or overlay ejournal is a type of open access academic journal, almost always an online electronic journal (ejournal), that does not produce its own content, but selects from texts that are already freely available online. While many overlay journals derive their content from preprint servers, others, such as the Lund Medical Faculty Monthly, contain mainly papers published by commercial publishers, but with links to self-archived preprint or postprints when possible.
The editors of an overlay journal locate suitable material from open access repositories and public domain sources, read it, and evaluate its worth. This evaluation may take the form of the judgement of a single editor or editors, or a full peer review process.
Public validation of subsequently approved texts may take several forms. At its most formal, the editor may republish the article with explicit approval. Approval might take the form of an addition to the text or its metadata. Or the editor may simply link to the article, via the table of contents of the overlay journal. An alternative approach is to link to articles already published in various open access ejournals, but adding value by grouping scattered articles together as a single themed issue of the overlay journal. Such themed issues allow the focussed coverage of relatively obscure or newly emerging topics.
In 2019, JMIR Publications, an open access publisher, announced the creation of a series of "superjournals", named JMIRx (JMIRx.org), which are overlay journals for preprint servers such as medRxiv, bioRxiv and PsyArXiv. 
The term 'overlay journal' was first coined by Paul Ginsparg in 1996. That same year, the journal Physical Review began to link to pre-prints that they had accepted, but not yet published. It was not until later that the first overlay journals were founded, including Journal of High Energy Physics, Logical Methods in Computer Science and Geometry and Topology, all of which were overlays for arXiv.
- Richard Van Noorden (17 January 2013). "Mathematicians aim to take publishers out of publishing". Nature. doi:10.1038/nature.2013.12243.
- Eysenbach, G (23 December 2019). "Celebrating 20 Years of Open Access and Innovation at JMIR Publications". Journal of Medical Internet Research. 21 (12): e17578. doi:10.2196/17578. PMC 6945123. PMID 31868653.
- Brown, Josh. "An introduction to overlay journals". UCL Discovery. Retrieved 24 March 2017.
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- Open Video Project: Overlay Journal prototype demonstration (2006)
- "Investigating overlay journals: introducing the RIOJA Project". D-Lib Magazine. September/October 2007
- Lund Medical Faculty Monthly
- Gibney, Elizabeth. "Open journals that piggyback on arXiv gather momentum". Nature. doi:10.1038/nature.2015.19102. Retrieved 30 August 2016.
- JMIRx (JMIRx.org)