Objective: Treatment of early-onset schizophrenia spectrum psychosis (EOS) is hampered by limited data on clinical presentation and illness course. We aimed to systematically review the clinical characteristics, diagnostic trajectories, and predictors of illness severity and outcomes of EOS. Methods: We conducted a systematic PubMed, PsycINFO, and Embase literature review including studies published from January 1, 1990 to August 8, 2014 of EOS patients with 1) ≥50% nonaffective psychosis cases; 2) mean age of subjects <19 years; 3) clinical samples recruited through mental health services; 4) cross-sectional or prospective design; 5) ≥20 participants at baseline; 6) standardized/validated diagnostic instruments; and 7) quantitative psychotic symptom frequency or severity data. Exploratory analyses assessed associations among relevant clinical variables. Results: Across 35 studies covering 28 independent samples (n = 1506, age = 15.6 years, age at illness onset = 14.5 years, males = 62.3%, schizophrenia-spectrum disorders = 89.0%), the most frequent psychotic symptoms were auditory hallucinations (81.9%), delusions (77.5%; mainly persecutory [48.5%], referential [35.1%], and grandiose [25.5%]), thought disorder (65.5%), bizarre/disorganized behavior (52.8%), and flat or blunted affect/negative symptoms (52.3%/50.4%). Mean baseline Positive and Negative Syndrome Scale (PANSS)-total, positive, and negative symptom scores were 84.5 ± 10.9, 19.3 ± 4.4 and 20.8 ± 2.9. Mean baseline Clinical Global Impressions-Severity and Children's Global Assessment Scale/Global Assessment of Functioning (CGAS/GAF) scores were 5.0 ± 0.7 and 35.5 ± 9.1. Comorbidity was frequent, particularly posttraumatic stress disorder (34.3%), attention-deficit/hyperactivity and/or disruptive behavior disorders (33.5%), and substance abuse/dependence (32.0%). Longer duration of untreated psychosis (DUP) predicted less CGAS/GAF improvement (p < 0.0001), and poor premorbid adjustment and a diagnosis of schizophrenia predicted less PANSS negative symptom improvement (p = 0.0048) at follow-up. Five studies directly comparing early-onset with adult-onset psychosis found longer DUP in EOP samples (18.7 ± 6.2 vs. 5.4 ± 3.1 months, p = 0.0027). Conclusions: EOS patients suffer substantial impairment from significant levels of positive and negative symptoms. Although symptoms and functioning improve significantly over time, pre-/and comorbid conditions are frequent, and longer DUP and poorer premorbid adjustment is associated with poorer illness outcome.