Abstract Purpose: Cergutuzumab amunaleukin (CA) is an immunocytokine comprising a variant form of interleukin 2 (IL2) [constructed to avoid CD25 binding and regulatory T-cell (Treg) stimulation] fused to a carcinoembryonic antigen (CEA)–targeted antibody. This phase Ib open-label, multicenter dose-escalation and -expansion study (NCT02350673) evaluated the safety, activity, pharmacokinetics, and pharmacodynamics of CA plus atezolizumab in patients with advanced/metastatic CEA-positive solid tumors. Patients and Methods: Patients received escalating doses of CA (6–20/25 mg) with fixed dosages of atezolizumab (840 mg) every 2 weeks or escalating dosages of CA weekly (10–15/20 mg) with fixed dosages of atezolizumab (1,200 mg) every 3 weeks. Primary objectives include maximum tolerated dose (MTD), recommended dose for expansion (RDE), and safety. Results: Twenty-four patients were randomized to receive CA plus atezolizumab every 2 weeks and 45 patients to CA weekly plus atezolizumab every 3 weeks. A subgroup of patients (n = 5) received obinutuzumab before treatment to study the prevention of antidrug antibodies. The MTD was not determined; 15 mg weekly or 20 mg every 2 weeks of CA plus atezolizumab was the RDE. The safety profile was consistent with CA monotherapy and atezolizumab-based therapies. The addition of atezolizumab did not affect the pharmacokinetic profile of CA, and treatment induced the proliferation of T and NK cells in the blood without Treg expansion. Increases in pharmacodynamic markers (C-reactive protein, lymphocytes, sCD25, and cytokines) suggested immune activation despite limited antitumor activity (overall response rate: 13.5% with weekly/every-3-week regimen). Conclusions: The safety profile of this combination was manageable. Prominent pharmacodynamic effects were elucidated; antitumor activity was limited.