The first Ocean Sampling Day (OSD) took place on June 21, 2014. In a coordinated effort, an internationally distributed group of scientists collected samples from marine surface waters in order to study microbial diversity on a single day with global granularity. Concurrently, citizen scientists enriched the OSD initiative through the MyOSD project, providing additional oceanographic measurements crucial to the contextualization of microbial diversity. Clear protocols, a user-friendly smartphone application, and an online web-form guided citizens in accurate data acquisition, promoting quality submissions to the project’s information system. To evaluate the coverage and quality of MyOSD data submissions, we compared the sea surface temperature measurements acquired through OSD, MyOSD, and automatic in situ systems and satellite measurements. Our results show that the quality of citizen-science measurements was comparable to that of scientific measurements. As 79% of MyOSD measurements were conducted in geographic areas not covered by automatic in situ or satellite measurement, citizen scientists contributed significantly to worldwide oceanographic data gathering. Furthermore, survey results indicate that participation in MyOSD made citizens feel more engaged in ocean issues and may have increased their environmental awareness and ocean literacy.