A new and distinct cultivar of Hydrangea macrophylla (Thunb.) named `True Blue` originated as a controlled cross between the cultivars `Blue Sky` and `Blue Danube`. The cultivar `True Blue` has blue--R.H.S. 100 A (blue group) sepals at maturity and is distinguished from other similar cultivars of which the inventor is aware by its sepal color, and its compact and sturdy growth habit. `True Blue` has large showy inflorescences, having both sepalous and non-sepalous florets, although at maturity the sepalous florets hide the non-sepalous florets.