Symphyotrichum lentum (formerly Aster lentus) is a species of aster known by the common name Suisun Marsh aster.
It is endemic to the marshes of Sacramento-San Joaquin River Delta of Northern California including the Suisun Marsh, in Solano County, for which it is named.
Symphyotrichum lentum is similar in appearance to Symphyotrichum chilense, which may be found in the same area. It is a colonial perennial herb producing a hairless stem 40 to 150 centimeters tall from a long rhizome. The leaves are linear or lance-shaped, pointed, and up to 15 centimeters long near the base of the plant. The lower leaves wither by the time the plant flowers.
The inflorescence is an open array of flower heads with a fringe of violet ray florets around a center of yellow disc florets. The fruit is a hairy achene with a long white pappus.