We report ultrasensitive Ca2+ indicators, yellow cameleon-Nano (YC-Nano), developed by engineering the Ca2+-sensing domain of a genetically encoded Ca2+ indicator, YC2.60 or YC3.60. Their high Ca2+ affinities (K-d = 15-140 nM) and large signal change (1,450%) enabled detection of subtle Ca2+ transients associated with intercellular signaling dynamics and neuronal activity, even in 100,000-cell networks. These indicators will be useful for studying information processing in living multicellular networks.