The solar‐driven photocatalytic reduction of CO2 (CO2RR) into chemical fuels is a promising route to enrich energy supplies and mitigate CO2 emissions. However, low catalytic efficiency and poor selectivity, especially in a pure‐water system, hinder the development of photocatalytic CO2RR owing to the lack of effective catalysts. Herein, we report a novel atom‐confinement and coordination (ACC) strategy to achieve the synthesis of rare‐earth single erbium (Er) atoms supported on carbon nitride nanotubes (Er1/CN‐NT) with a tunable dispersion density of single atoms. Er1/CN‐NT is a highly efficient and robust photocatalyst that exhibits outstanding CO2RR performance in a pure‐water system. Experimental results and density functional theory calculations reveal the crucial role of single Er atoms in promoting photocatalytic CO2RR. A catalyst with a high density of rare‐earth single Er atoms supported on a carbon nitride nanotube (HD‐Er1/CN‐NT) is synthesized by an atom‐confinement and coordination strategy (ACC). Experimental results and DFT calculations reveal that the single Er atoms play a key role in the photocatalytic CO2 reduction reaction in a pure‐water system. The ACC strategy also extends to the synthesis of other rare‐earth single‐atom catalysts.