The positive effects of mindfulness-based interventions (MBIs) on mental health have been reported in diverse populations. However, few studies have examined the relationship between MBIs and biomarkers. The aim of this systematic review was to examine the effects of MBIs on specific biomarkers in patients with psychiatric disorders. Methods: This systematic review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA, [1]) guidelines. We conducted a systematic computerized literature search of PubMed, EMBASE, PsycINFO and the Cochrane library. The reference lists of the identified original articles and reviews were also searched manually for additional studies. Risk of bias in the included studies was assessed using the Cochrane Collaboration's tool [2]. The protocol was registered with the International Prospective Resister of Systematic Reviews (PROSPERO, [3]) (registration number CRD42018084768). Results: Of the initial search of 149 records including 19 duplicates, 119 were excluded after screening of the abstracts and full texts of the remaining 11 articles were assessed. We finally selected eight studies with 916 participants (573 women, 62.6%): 596 patients with psychiatric disorder and 320 healthy controls. Six were randomized controlled trials (RCTs), and the remaining two were open-label studies. Five studies focused on depression and/or anxiety disorder, one focused on alcohol dependence, one on attention-deficit hyperactivity disorder (ADHD), and one on sleep disturbance. According to the Cochrane risk of bias tool [2], four studies were considered to have a ‘low risk’ of bias, being of high quality. The biomarkers mainly used blood samples while three studies used salivary and electroencephalogram (EEG) samples. Interleukin 6 (IL-6) and cortisol were the most frequently examined in three studies, followed by C-reactive protein (CRP) in two studies. MBIs included largely two types: mindfulness-based stress reduction (MBSR) and mindfulness-based cognitive therapy (MBCT). The other studies were conducted with modified MBSR. Due to the heterogeneity of biomarkers and participants, and the insufficient quality of the included articles, a quantitative synthesis was not possible. MBIs showed some effects on biomarkers in four studies, and statistically significant findings were found in two RCTs. One study showed that MBCT group significantly enhanced error positivity (Pe) and inhibitory control (NoGo-P3) amplitudes (p = 0.02 and p =0.02, respectively) compared to wait-list group in patients with ADHD; Increased Pe amplitudes correlated with a reduction in hyperactivity/impulsivity symptoms, and enhanced NoGo-P3 amplitudes were associated with an improvement in inattention symptoms. The other study showed that MBSR group had a significantly greater reduction in adrenocorticotropic hormone (ACTH), tumor necrosis factor (TNF)-alpha, and IL-6 Area Under the Curve (AUC) (p = 0.007, p = 0.036, p = 0.034, respectively) compared to control group in patients with generalized anxiety disorder (GAD). Conclusions: Our findings suggest that MBIs may have a positive effect on some biomarkers in patients with psychiatric disorders, particularly ADHD and GAD. Further trials are needed to establish the impact of MBIs on biomarkers in patients with psychiatric disorders.