The aim was to compile the evidence from Randomized Controlled Trials (RCTs) of diet or dietary supplements used to reduce disease activity in adults with Rheumatoid Arthritis (RA). Searches were performed in the databases PubMed, Scopus and Cochrane. Only RCT studies of diets, foods or dietary supplements, looking at effects on the Disease Activity Score in 28 joints (DAS28) among adults with RA, published in peer-reviewed journals, were included. A total of 27 articles were included—three of whole diets (Mediterranean diet, raw food and anti-inflammatory diet), five of food items, five of n-3 fatty acids, five of single micronutrient supplements, four of single antioxidant supplements and five of pre-, pro- or synbiotics. Studies that showed moderate strength evidence for positive effects on disease activity in RA included interventions with a Mediterranean diet, spices (ginger powder, cinnamon powder, saffron), antioxidants (quercetin and ubiquinone), and probiotics containing Lactobacillus Casei. Other diets or supplements had either no effects or low to very low strength of evidence. In conclusion, RCT studies on diet or dietary supplements are limited in patients with RA, but based on the results in this review there is evidence that some interventions might have positive effects on DAS28.