Introduction: Anger is a state of emotions ranging from irritation to intense rage. Aggression implies externalizing anger through destructive/punitive behaviour. The World Professional Association for Transgender Health (WPATH) Standards of Care, Edition 7 (SOC7) guidelines warn about aggression in transgender men (TM) on testosterone treatment. We aimed to assess whether anger intensity increases in TM and decreases in transgender women (TW) after initiation of gender affirming hormone therapy and to identify predictors for anger intensity in transgender people. Methods: This prospective cohort study was part of the European Network for the Investigation of Gender Incongruence (ENIGI). Anger intensity was prospectively assessed in 898 participants (440 TM, 468 TW) by STAXI-2 (State-Trait Anger Expression Inventory-2) State Anger (S-Anger) during a three-year follow-up period, starting at the initiation of hormone treatment. Data were analysed cross-sectionally and prospectively. Results: There was no change in STAXI-2 S-Anger scores. At three, twelve and thirty-six months of gender affirming hormone therapy, STAXI-2 S-Anger scores were not correlated to serum testosterone levels, although there was a correlation with various psychological measures after three and twelve months. TM experiencing menstrual spotting after three months had higher STAXI-2 S-Anger scores compared to those without (median 26.5 [18.0–29.8] versus 15.0 [15.0–17.0], P = 0.020). Changes in STAXI-2 S-Anger scores were not correlated to changes in serum testosterone levels after three, twelve and thirty-six months in TM or TW. Conclusions: State-level anger intensity is associated with psychological and/or psychiatric vulnerability, but not exogenous testosterone therapy or serum testosterone levels in transgender people.