To improve the results of the classic periodontal treatment, probiotics have been suggested recently to decrease the number of bacteria and the expression of mediators of inflammation. This systematic review aimed to assess the literature for the effectiveness of different probiotic strains as adjuvants to non-surgical periodontal therapy. The electronic database of MEDLINE (via Pubmed) was searched up to December 2017 for randomised clinical trials in English comparing non-surgical periodontal treatment and probiotics versus periodontal treatment and placebo. The primary outcome investigated was reduction in pocket probing depth. Secondary outcomes were bleeding on probing, plaque index reduction and bacteria counts. Nine trials were included. A narrative data synthesis did not result in any major improvement of overall pocket probing depth but moderate pockets from 4 to 6 mm showed larger reductions in study groups, which could decrease the need for surgery. Sites with bleeding on probing and presence of plaque decreased after treatment. For periimplant mucositis, there was a small tendency to better results in the study group. With the available data, it is concluded that probiotics may provide an additional benefit to manual debridement in chronic periodontitis. More studies are required on dose, route of administration and strains of probiotics used.