Description: The yield of rice ( Oryza sativa L.) has increased substantially with the development of new cultivars, but the role of potassium (K) requirement for the increase in grain yield and the genotypic advance is still unclear. In order to investigate this relationship a database of 1199 on-farm measurements (harvest index 0.4) comprising 〉 400 modern rice cultivars was collected during 2005–2010 across major irrigated lowland rice–production regions of China. This was used to evaluate the relationships among K requirement, grain yield, and genetic improvement. Across all the sites and seasons, mean reciprocal internal efficiency of K (RIE-K, kg K [t grain produced] –1 ) was 19.8 kg K (t grain) –1 and rice yield averaged 8.7 t ha –1 . Considering four levels of grain yield (〈 7.5, 7.5–9, 9–10.5, and 〉 10.5 t ha –1 ), the respective RIEs were 18.7, 19.4, 20.5, and 21.7 kg K (t grain) –1 . The gradual increase in the RIE-K with yield was attributed mainly to the increase in straw and grain K concentration and the decrease in the K harvest index. The RIE-K values for ordinary inbred, ordinary hybrid, and “super rice” were 18.5, 20.1, and 19.9 kg K (t grain) –1 , respectively. Examining the historical development of rice cultivars, the RIE-K decreased from 40.9 (Nanjing1, early tall, inbred) in the 1950s to 19.8 (IR24, semi-dwarf, ordinary inbred) in the 1970s, and then increased to 20.9 (Shanyou63, modern ordinary hybrid) in the 1980s and 20.6 kg K (t grain) –1 (II-you084, “super” rice) in the 2000s. This variation in RIE-K among grain-yield levels and cultivars highlights the importance of information on rice K requirement in calculating K balance and optimal K-fertilizer rate for rice production.