Earthworms are postdispersal seed predators that can influence weed communities in temperate agroecosystems. Recent studies have found that seed feeding by earthworms tends to be driven by the active selection of certain seed species rather than random encounter. Numerous seed traits are expected to affect seed selection by earthworms, including seed size, shape, coat hardness, and nutritional content. The impact of these traits on seed selection by earthworms tends to vary depending on seed species identity and earthworm species identity, rendering the outcome of earthworm–seed interactions hard to predict. We carried out laboratory experiments to investigate the impact of seed physical and chemical traits on seed choice by the common earthworms (Lumbricus terrestris). Seeds of six weed species typical of the Northern Great Plains agroecosystem, wild mustard (Sinapis arvensis L.), field pennycress (Thlaspi arvense L.), shepherd’s purse [Capsella bursa-pastoris (L.) Medik.], catchweed bedstraw (Galium aparine L.), green foxtail [Setaria viridis (L.) P. Beauv.], and redroot pigweed (Amaranthus retroflexus L.), were offered to L. terrestris in multiple-choice feeding arenas. The results showed that seeds of S. arvensis and C. bursa-pastoris, both of which have high lipid content, were the most consumed. Seed ingestion was negatively influenced by irregular seed shapes and long seed length, but these physical traits did not override the strong preference for lipid-rich seeds. These findings suggest that seed selection by L. terrestris earthworms was strongly influenced by the lipid content of the seed when seed morphology (i.e., size and shape) varied within certain limits. Therefore, seed nutrients are likely to play an important role in weed seed choice by L. terrestris earthworms when seed physical traits do not impose major constraints on ingestion.