Cyanobacterial harmful algal blooms (cyanoHABs) are a growing problem in aquatic environments. Here, a new data buoy system is described (i.e., “the Panther buoy”) for monitoring cyanoHABs and associated conditions. The system uses a custom open-source data logger, an embedded modem, and a solar charge regulator with off-the-shelf sensors on a compact platform. Panther buoys were deployed in Green Bay, USA, and Lake Superior at the Apostle Islands, USA, to monitor water temperature with depth and chlorophyll-a and phycocyanin fluorescence. Additional Panther buoys were deployed in Sturgeon Bay and Summerfest Lagoon, Milwaukee, to monitor wind and water temperature with depth. Cellular data were transmitted every 2–10 min in 98 ± 10% of data transmissions of 10–30K sampling events per deployment. A new temperature chain is described and shown to have high precision (R = 0.99 and P < 0.001) and accuracy within 0.31 °C of commercial sensors. Panther buoys in Lake Superior and Green Bay detected increases in phycocyanin or chlorophyll-a fluorescence, indicative of moderate bloom events. Wind measurements in Summerfest Lagoon and Sturgeon Bay were significantly correlated with commercial wind sensors. Altogether, these deployments indicate that the Panther buoy system is a reliable device for monitoring cyanoHABs and associated conditions.