Description: During the late Miocene, global cooling occurred alongside the establishment of near-modern terrestrial and marine ecosystems. Significant (3 to 5 °C) sea surface cooling from 7.5 to 5.5 Ma is recorded by proxies at mid to high latitudes, yet the magnitude of tropical cooling and the role of atmospheric carbon dioxide (pCO2) in driving this trend are debated. Here, we present a new orbital-resolution sea surface temperature (SST) record spanning the late Miocene to earliest Pliocene (9 to 5 Ma) from the eastern equatorial Indian Ocean (International Ocean Discovery Program Site U1443) based on Mg/Ca ratios measured in tests of the planktic foraminifer Trilobatus trilobus. To test if an atmospheric pCO2 decrease may have driven this cooling, we also present new paleoclimate model simulations under three atmospheric pCO2 scenarios (300 ppm, 420 ppm and 560 ppm; in the range suggested by existing pCO2 proxy records). The data contains Mg/Ca ratios (mmol/mol) measured in tests of a planktic foraminifera species living in the mixed layer (Trilobatus trilobus), together with reconstructed Sea Surface Temperatures. Samples are from the revised shipboard splice from Site U1443 (equatorial Indian Ocean) retrieved during International Ocean Discovery Program Expedition 353. Data span the late Miocene-earliest Pliocene (9 to 5 Ma) with a mean resolution of 5.5 kyr and allow reconstruction of SSTs on secular to orbital timescales. Sea Surface Temperatures are reconstructed with the Dekens et al., (2002) T. sacculifer calibration equation from Pacific Ocean, including a basin-specific dissolution correction. Correction for variation of Mg/Ca ratio of seawater are also calculated following the scenario from Higgins & Schrag (2015) and the approach from Tierney et al. (2019). ±1σ and ±2σ uncertainties linked to analytical, calibration, and age model errors are estimated via a bootstrap Monte Carlo procedure using the Paleo-Seawater Uncertainty Solver (PSU solver, Thirumalai et al., 2016) in Matlab.