This Remotely Operated Vehicle (ROV) shakedown expedition aboard NOAA Ship Okeanos Explorer began in Norfolk, Virginia on June 13, 2021 and concluded on June 28, 2021 in Newport, Rhode Island. This was the first time the ROVs Deep Discoverer and Seirios were mobilized and used for ROV dives since the extensive 2021 winter dry dock repair period. While underway, 24‐hour operations focused on preparing remotely operated vehicle (ROV) systems for the remainder of 2021 expeditions. Extensive testing, calibration, and troubleshooting was completed for new ROV motors, motor controllers, cameras, lighting, hydraulic systems, and a navigational sonar during 11 ROV dives. The first five dives progressively got to deeper depths ranging from 670 m to 4370 m with the primary objective of new equipment integration and personnel training. The following five dives were conducted on Caryn Seamount, and the mid-Atlantic canyons Toms, Hudson (twice), and Uchupi. The last dive of the expedition was on a recently discovered sonar anomaly that turned out to be a World War II era submarine, the Humaitá (ex-USS Muskallunge). Five biological and three geological samples were collected for later analyses from Caryn Seamount and the mid-Atlantic canyons. Mapping operations during the expedition included continued calibrating and troubleshooting of the ship's new EM304 multibeam sonar and transducer array. Additional mapping operations included improving multibeam bathymetry in previously mapped areas, sub-bottom sonar surveys conducted on the Currituck landslide feature off of North Carolina, EK60 water column sonar survey of the diurnal migration above Hudson Canyon, and searching for underwater cultural heritage sites by observing sonar depth and backscatter anomalies.