Will Canon's EF-EOS R adapter work with the new EOS R5 or will I need something else?

You can look at the official Canon EOS R5 User Guide (PDF) for information. Download the guide from the Canon website. There are over 900 pages but many live links so you can easily jump around inside the document. Go to page 878 and there you should see a chart that shows all current items that can be connected to the camera. One thing you may like to consider is that, though you can connect EF lenses, not all of them will provide access to all the features of the camera. For example, focussing speed will not necessarily be as fast as with the dedicated RF series lenses. I hope this is useful for you.

When that product specification page was written, the EOS R was the only RF mount camera available. The RP, R5, and R6 hadn't been introduced yet.

The same EF→RF adapters that work with EF lenses for the EOS R and EOS RP will also work with the newer R5 and R6.

In the EOS R5 Specifications document the following two entries are listed as Compatible Lenses:

• Canon RF lens group (excluding EF, EF-S and EF-M lenses)
• When using Mount Adapter EF-EOS R: Canon EF or EF-S lenses (excluding EF-M lenses)

There are currently four varieties of Mount Adapter EF-EOS R available:

• Mount Adapter EF-EOS R is a straight adapter with no additional features
• Control Ring Mount Adapter EF-EOS R includes a control ring as found on RF lenses
• Drop-In Filter Mount Adapter EF-EOS R with Drop-In Circular Polarizing Filter A
• Drop-In Filter Mount Adapter EF-EOS R with Drop-In Variable ND Filter A

I am sure the product specification page has not been updated since the launch and which is why it is mentioning the camera bodies available at around that time. There is no reason for the rf-ef adapter to not work since they communicate via the same protocol/mount. If that would have been different for each camera (not consistent across all cameras with the same mount) then future lenses would also not work with previous cameras of the same mount!

So I am sure, unless Canon puts out something like a compatibility matrix of lenses/adapters vs cameras, all rf mount lenses/adapters will continue to work with all rf mount bodies.

That is the whole point of standards and good designs - i think the latest EF lenses continue to work as designed with the EF film bodies!

Hope that helps!