Will this plate also fit a Nikon D300?
Yes, it should fit. The diagram of the plate on the web page lists the D300. These plates are fairly universal, in terms of the plate itself. The important thing to know is the size of the threading that the plate secures to (the female slot on the camera body). That should be fairly easy to find out. As for the dimensions of the plate in relation to the camera body, I'd look up specs on the D700/D3 since this plate is listed for those models, and see how your D300 compares.