The main purpose of this article is to assess mutual funds as instruments for additional pension security on the example of Poland. In the paper, we characterized the operation of the third pillar of the Polish pension system, with particular emphasis on individual pension accounts and individual pension security accounts. In addition, we used the Sharpe ratio and carried out an assessment of investment efficiency of the mutual funds available under the third pillar for the period of 2009-2019. We also performed the Kruskal-Wallis test, finding that the type of fund had a significant impact on investment performance. Moreover, the post-hoc test showed that the highest investment efficiency is achieved by fixed income funds available under the third pillar.