Mounting sustainability pressures challenge established firms to engage with sustainability innovations, which are often introduced by startups. Research on alliance learning has established the potential of learning from startups to advance corporate innovation. Here, scholars have outlined alliance learning processes and outcomes and have distinguished learning about and learning from alliance partners as two key learning types. The saliency of learning from the operational alliance process is stressed. To date, however, no study has investigated alliance learning processes and outcomes for sustainability innovations. This is despite the fact that sustainability research suggests learning processes in the sustainability context have a distinct nature. This study addresses this research gap by analyzing the sustainability-specific learning processes and outcomes of a large European meat producer and wholesaler with a turnover of $2.7bn in the fiscal year 2019/2020. The firm formed alliances with nine startups for sustainable plant, insect-based and cell-based protein solutions. Our analysis (1) identifies three distinct characteristics of sustainability-related alliance learning processes and outcomes, and (2) specifies the temporal occurrence and outcomes of learning types in alliance learning phases. In contrast to findings of prior research, our study reveals that learning about alliance partners is of key importance throughout the whole sustainability-oriented alliance learning process. In addition, the findings highlight that alliance learning outcomes may support an established firm's contribution to the sustainability transformation of mass markets.