Pseudogenes have been reported to exert oncogenic or tumor-suppressive functions in cancer. However, the expression, role, and mechanism of pseudogene-derived RNAs in breast cancer remain unclear. The RNA levels and prognostic values of pseudogenes in breast cancer were determined. The levels of RP11-480I12.5 in cell lines and clinical samples were validated by quantitative real-time PCR. In vitro effects of RP11-480I12.5 on cell growth were measured by cell counting kit-8 (CCK-8) assay, colony formation assay, cell counting assay, and flow cytometry analysis. Xenograft model was established to detect its in vivo effect. The potential mechanism of RP11-480I12.5 was also studied by a combination of bioinformatic analysis and experimental confirmation. Finally, the possible functional parental genes of RP11-480I12.5 in breast cancer were explored. After a series of bioinformatic analyses, RP11-480I12.5 was selected as the most potential pseudogene in breast cancer. RP11-480I12.5 expression was significantly upregulated in breast cancer cell lines and clinical breast cancer tissues. Knockdown of RP11-480I12.5 markedly suppressed cell proliferation and colony formation, induced cell apoptosis of breast cancer in vitro, and inhibited tumor growth in vivo. Four transcripts of RP11-480I12.5 (001/002/003/004) were identified. Only overexpression of RP11-480I12.5-004 significantly enhanced cell growth of breast cancer both in vitro and in vivo. RP11-480I12.5-004 mainly located in cytoplasm and increased AKT3 and CDK6 mRNA expression, at least in part, by competitively binding to miR-29c-3p. Six parental genes of RP11-480I12.5 were found, among which TUBA1B and TUBA1C were statistically linked to RP11-480I12.5 expression, possessed prognostic values, and were upregulated in breast cancer. Our findings suggested that pseudogene-derived long non-coding RNA (lncRNA) RP11-480I12.5-004 promoted growth and tumorigenesis of breast cancer via increasing AKT3 and CDK6 expression by competitively binding to miR-29c-3p.