Yes...there is a seal.
Try cleaning it first and the grove/track that it goes into when closed. Might try something like Vaseline on the existing seal also. Maybe just some debris/toilet paper..but.
If this fails you will have to remove the toilet and replace the seal from underneath or by taking the toilet apart...That's what I had to do. RV dealers have the part...also on line.
Not all that difficult (and not all that much fun either).
Google your toilet brand and model and you can download the step by step instructions..There are also videos on youtube.
Good luck with it.