Earth's climate has oscillated between short-lived interglacial and extended glacial periods for the past million years. Before the last interglacial, absolutely dated markers of sea level become increasingly rare; hence, our knowledge of sea-level change driven by the waxing and waning of continental ice sheets before that time is largely based on proxy records from deep-sea cores(1-3) that lack direct age control. Here we present precise U-Th ages for a remarkable collection of submerged speleothems(4,5) from Italy, which record three sea-level highstands during the penultimate interglacial period, Marine Isotope Stage 7, from 245,000 to 190,000 years ago. We find that sea level rose above-18m (relative to modern sea level) several thousand years before maximum Northern Hemisphere insolation during the first and third highstands. In contrast, the second highstand, Marine Isotope Stage 7.3, is essentially synchronous with the insolation maximum, and sea level during this highstand only peaked at about 18 m, even though the concurrent insolation forcing was the strongest of the three highstands. We attribute the different phasing and amplitude of the Marine Isotope Stage 7.3 highstand to the extensive continental glaciation that preceded it. This finding highlights the significance of cryosphere response time to the climate system.