A drilling riser adapter variably connects and releases a riser from a subsea wellhead assembly. The drilling riser adapter has a hydraulically actuated engagement assembly for selectively engaging and disengaging a lower end of the marine riser. The drilling riser adapter also includes a control panel communicatively coupled to the engagement assembly for actuating the engagement assembly to engage and disengage the lower end of the marine riser. The drilling riser adapter also includes a hydraulic fluid pressure receptacle on the control panel for engagement by a remotely operated vehicle to supply hydraulic fluid pressure to the engagement assembly. The drilling riser adapter may be actuated subsea to release a first riser from the wellhead assembly, and connect to a second riser.