Mean sea level observations from an historical tide gauge located in Cadiz (Southern Spain) spanning the period 1880–1924 were recovered from national archives. Daily sea level averages stored in handwritten log books were digitized, quality controlled, and referred to the same benchmark. A careful analysis of all the high precision leveling surveys available in the area of the tide gauge enabled the establishment of a common datum with a modern record starting in 1961 from another tide gauge located only 2.5 km apart, with accuracy better than 5 mm. As a result, a consistent daily mean sea level record from 1880 to 2009 was constructed. The 20th century relative mean sea level rise in Cadiz is 0.7 ± 0.1 mm yr−1, which becomes 1.0 ± 0.2 mm yr−1 once corrected for vertical land movement with high precision GPS data, in agreement with nearby records. The analysis of the seasonal sea level cycle indicated that the amplitude of the annual cycle has increased during the 20th century. This work evidences the significance of sea level data rescue for present-day climate research.