BE58 pilot reported the right-hand passenger slightly opened on departure; but fully opened and got stuck in that position after climbing to a higher altitude. The pilot then performed an air turnback. The cause of the door opening inflight may have been due to the seatbelt getting stuck in the door; even though the pilot was able to get the door seemingly closed and secure before takeoff.

Narrative

On departure from ZZZ; the right-hand side passenger door came open. At first; the door only opened slightly and noise was the only factor. Nothing changed in the way of flying the airplane. I got the hand-off from Tower to ZZZ Departure; and during my initial check-in out of 1;500 ft. climbing to 2;000 ft.; the door swung fully open and became stuck in that position. At this point; the airplane did not fly in a stable matter. There was so much noise that I could no longer hear ATC with much clarity. At this point; an immediate return to ZZZ was needed; and I had already informed Departure that we were going to need to come back to the airport. I knew we were moving away from the airport and I did not know how much longer I was going to be able to fly the airplane; and if the door was going to rip off and cause structural damage to the airplane. I [requested priority handling] and said we were going to proceed straight in for Runway XX at ZZZ; and to send the trucks if possible; as I did not have confidence in the aircraft performance at low speed. Once I landed the airplane; I determined there was no longer a need to continue with the [priority handling] and I advised Tower that we were no longer an immediate aircraft and assistance was no longer required. I had a passenger in the aircraft on the previous flight; which had terminated only minutes before the other one commenced. The seat belt was stuck in the door; and I was able to put the door handle into the closed position getting a positive indication that the door was closed and secure. I should have looked at the entire door to confirm that it was closed and secure; and not just the general frame and the door closed indicator. I will now be doing that on every flight in this aircraft in the future. At the end of the day; I; as PIC (Pilot in Command); am responsible for all the doors being secured; not any passenger. This was nobody's fault other than my own.