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Small regulatory RNAs (sRNAs) are post-transcriptional regulators of bacterial gene expression. In cyanobacteria, the responses to nitrogen availability, that are mostly controlled at the transcriptional level by NtcA, involve also at least two small RNAs, namely NsiR4 (nitrogen stress-induced RNA 4) and NsrR1 (nitrogen stress-repressed RNA 1). Prediction of possible mRNA targets regulated by NsrR1 in <i>Nostoc</i> sp. PCC 7120 allowed, in addition to previously described <i>nblA</i>, the identification of <i>all1871</i>, a nitrogen-regulated gene encoding a protein of unknown function that we describe here as required for growth at the expense of atmospheric nitrogen (N₂). We show that transcription of <i>all1871</i> is induced upon nitrogen step-down independently of NtcA. All1871 accumulation is repressed by NsrR1 and its expression is stronger in heterocysts, specialized cells devoted to N₂ fixation. We demonstrate specific interaction between NsrR1 and the 5′ untranslated region (UTR) of the <i>all1871</i> mRNA, that leads to decreased expression of <i>all1871</i>. Because transcription of NsrR1 is partially repressed by NtcA, post-transcriptional regulation by NsrR1 would constitute an indirect way of NtcA-mediated regulation of <i>all1871</i>.