microRNAs (miRNAs) are endogenous non-coding small RNAs (sRNA) that play important regulatory functions in growth, development, and environmental stress response of eukaryotes. Currently, miRNA-size sRNA (msRNAs) was discovered in several prokaryotes through deep sequencing. However, no data are available on whether msRNAs exist in cyanobacteria and whether they regulate biological tolerance to salt stress. In this study, three small RNA libraries were constructed from control (0.02 M NaCl), medium- (0.3 M NaCl), and high-salt treatments (0.5 M NaCl) of Spirulina platensis. After sequencing using a high-throughput Illumina Solexa system, nine msRNAs with msRNA* were identified, and 21 candidate msRNAs showed significantly differential expression under salt-stress conditions. Seven of the selected msRNAs presented the consistent expression trends when compared with their deep sequencing results as verified by quantitative real-time polymerase chain reaction (qRT-PCR), demonstrating that the expression analyses for msRNAs according to small RNA sequencing data were reliable. Through computational identification, 33 target genes were predicted for 12 msRNAs in S. platensis; Gene Ontology (GO) and KEGG enrichment analyses revealed that the putative target genes were grouped into the categories of proteolysis, protein homotrimerization, glycosylation, ubiquinone biosynthetic process, DNA restriction-modification system, and polysaccharide catabolic process. Using proteomic analysis, two target proteins (glyceraldehyde-3-phosphate dehydrogenase and forkhead-associated (FHA) domain-containing protein) were differentially expressed under salt-stress conditions, which might be regulated by msRNAs. Our study demonstrates that msRNAs exist in S. platensis, and these msRNAs may have an important role in salt-stress responses; however, their functional significance requires further investigation.