I have a GRX RD-RX810 rear derailleur and I need a compatible (thumb) shift lever for flat bars.
Any suggestions which shifters are compatible for my setup?
The other answer has already a specific product, but i wanted to answer in a more generic way for people with a similiar issue and maybe not the exact same product :)
You can find official compabilitly list of all shimano components at
You need to know which specific version you have. (not just RX810, but also 1x11/2x11 etc.)
In the compability chart, look for Shifters with the SL prefix.
SL = flat handlebar shifters
ST = drop handlebar shifters
For your case, you most likely you can use shifters from the 105 series and there was a limited edtion GRX specific flat handlebar shifters.
Shimano offers thumb shifters that are compatible with 11-speed road groupsets
I haven't checked if there is a newer iteration of the product since 105 R5800 is out for a few years but they've been used on flatbar GRX bikes, as an example: https://www.cube.eu/ch-en/cube-nulane-race-fe-rubyred-n-black/cb9372bb50c65be2c045a3ed7996cbb7
Edit As out also mentioned in the other answer, scrolling to the 11-speed section of the road rear drivetrain compatibility chart lists these (and SL-U5000-R) as the current options. SL-prefixed components is what you have to generally look for. ;)