Many 3D modeling and CAD tools use .prj files to store various 3D/CAD projects and other work in progress. So it may be very well possible that some of the .prj files can be exported and saved as 3D Objects (.obj).
Convert Project file to 3D object graphics.
We found 4 software records in the database related to .prj to .obj file format conversion.
Microsoft Windows software - convert prj to obj on Windows
A widely-used 3D animation application from Autodesk
Convert from prj file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned prj to obj. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A graphics design tool for making 3D landscaping and animations
Apple macOS / Mac OS X software - convert prj to obj on OS X
3D modeling and animation package for Mac.
On-line services software - convert prj to obj on web
A free online 3D model conversion service