Introduction: Osteoarthritis (OA) is the most common disabling joint disease worldwide. Moreover, its incidence and prevalence are increasing because of aging, higher life expectancy, and lifestyle changes, leading to a growing population of patients with OA. Current treatment of OA includes nonpharmacologic and pharmacologic modalities. Phytothermotherapy (PTT) is a singular crenotherapic treatment consisting of immersing oneself in pools of fermenting alpine grass, to exploit its heat and rich, aromatic components. The efficacy of PTT in OA is bolstered by ancient tradition. However, there is a marked lack of clinical validation of its efficacy and tolerability in current literature. The aim of this review was to evaluate the currently available knowledge of possible clinical effects and mechanisms of action of PTT in OA. Methodology: We searched PubMed and Scopus (the period examined was 1980-2012) for clinical trials examining the effect of PTT in OA. MEDLINE was searched using the term "phytothermotherapy" and "hay baths" in combination with "osteoarthritis." We included only papers published in English or Italian and in peer-reviewed journals. Results: We identified three trials describing the results of PTT in OA. The available data demonstrate that PTT is efficacious in decreasing pain and disability and improving function in patients with OA. Moreover, the tolerability of PTT is excellent. The mechanisms of action of PTT are not yet completely known, although it is probably due to different combined mechanical, physical, and chemical effects. Discussion: PTT could represent a useful aid in the treatment of OA or a valid alternative for patients who do not tolerate pharmacologic treatments. However, further studies on a larger number of patients are needed to provide more precise therapeutic guidelines on the modalities of use of PTT. Additionally, there is a need for other botanical investigations and research on the mechanisms of action of PTT.