Most high-resolution, proxy-based paleoclimate research has concentrated on tropical oceans, while mid- and high-latitude marine regions have received less attention, despite their importance in the global climate system. At present, sclerochronological analyses of bivalve mollusks supply the bulk of annual- to subannual-resolution extratropical marine climate data, even though interpretation is complicated by a slowdown of growth with increasing shell age. Hence, in order to address the need for additional high-resolution proxy climate data from extratropical regions, we conducted the first year-long in situ field calibration of the coralline red alga Clathromorphum compactum in the Gulf of Maine, United States. Coralline red algae are widely distributed in coastal regions worldwide, and individual calcified plants can live continuously for several centuries in temperate and subarctic oceans. Stable oxygen isotopes extracted at subannual resolution from growth increments of monitored specimens of C. compactum relate well to in situ–measured sea-surface temperatures during the May to December calcification period, highlighting the suitability of coralline red algae as an extratropical climate archive. Furthermore, there is a strong correlation between a 30 yr σ18O record of C. compactum and an instrumental sea-surface temperature record (r = −0.58, p = 0.0008) and a proxy reconstruction derived from the bivalve Arctica islandica collected in the central Gulf of Maine (r = 0.54, p = 0.002).