What documents do I need to exit Thailand and enter Laos with a car that I do not own?

To leave Thailand and enter Laos with a car that is not yours, you will need a passport and a declaration from the car's owner granting you permission to use the vehicle. Additionally, if the car is under financing, you will need approval from the financing company. Many commenters suggest that transporting a vehicle not owned by you can be cumbersome and may not even be feasible, advising travelers to consider crossing the border without a vehicle and renting one on the Laotian side instead.