MicroRNAs (miRNAs) are small noncoding RNAs that post-transcriptionally regulate gene expression. There have been several reports of miRNA deregulation in prostate cancer (PCa) and the biological evidence for an involvement of miRNAs in prostate tumorigenesis is increasing. In this study, we show that miR-34c is downregulated in PCa (p = 0.0005) by performing qRT-PCR on 49 TURPs from PCa patients compared to 25 from patients with benign prostatic hyperplasia. The miR-34c expression was found to inversely correlate to aggressiveness of the tumor, WHO grade, PSA levels and occurrence of metastases. Furthermore, a Kaplan–Meier analysis of patient survival based on miR-34c expression levels divided into low (< 50th percentile) and high (> 50th percentile) expression, significantly divides the patients into high risk and low risk patients (p = 0.0003, log-rank test). The phenotypic effects of miR-34c deregulation were studied in prostate cell lines, where ectopic expression of miR-34c decreased cell growth, due to both a decrease in cellular proliferation rate and an increase in apoptosis. In concordance to this, miR-34c was found to negatively regulate the oncogenes E2F3 and BCL-2, which stimulates proliferation and suppress apoptosis in PCa cells, respectively. Reversely, we could also show that blocking miR-34c in vitro increases cell growth. Further, ectopic expression of miR-34c was found to suppress migration and invasion. Our findings provide new insight into the role of miR-34c in the prostate, exhibiting tumor suppressing effects on proliferation, apoptosis and invasiveness.