The blast occurred inside one of the rooms on the third floor.
One employee was seriously hurt and a visitor sustained minor injuries, officials say.
More than a dozen patients felt the blast and heard a loud boom.
They were all moved to rooms on other floors.
The explosion is believed to be accidental and related to an oxygen tank.
Princeton Healthcare System released the following statement Friday afternoon:
"University Medical Center of Princeton at Plainsboro (UMCPP) is continuing to cooperate with the ongoing investigation into the explosion that occurred last night, which is believed to be accidental in nature. The employee who was seriously injured has been transferred to a burn center in the region. The visitor who sustained minor injuries has been treated and released. Counselors and chaplains are providing emotional support to patients, family members and employees. The hospital is continuing with normal operations for inpatients and outpatients today, including surgeries and outpatient services such as Lab and Radiology."
The investigation into the incident is ongoing.