Inclusion Criteria:  Written informed consent consistent with International Council for Harmonisation Good Clinical Practice guidelines and local laws signed prior to entry into the study and any study-related procedures Male or female participants aged ≥ 18 years at the time of informed consent  For both the Phase 1b and 2 portions of the study, participants must have SSc (with or without ILD), as defined using the American College of Rheumatology/European League Against Rheumatism criteria  For the Phase 1b study, participants must have diffuse SSc (with and without ILD) For the Phase 2 study, participants must have SSc-ILD. Presence of ILD will be confirmed by central reading of the screening high-resolution computed tomography (HRCT) SSc disease onset (defined by first non-Raynaud symptom) must be within 60 months of screening If participant is on a non-excluded immunosuppressive therapy (e.g. mycophenolate, methotrexate, azathioprine, etc.) the dose should be stable for > 6 months at the time of screening FVC ≥ 50% of predicted normal at screening  Presence of at least one of the following at screening:  C-reactive protein levels at screening of ≥ 6 mg/L Erythrocyte sedimentation rate ≥ 28 mm/hr Platelet count ≥ 330 × 10^9/L (33,000/μL) mRSS at screening of ≥ 15 DLCO (corrected by hemoglobin at screening): ≥ 40% to ≤ 89%  Women of childbearing potential must:  Have 2 negative pregnancy tests as verified by the investigator prior to starting study therapy. She must agree to ongoing pregnancy testing during the course of the study and until 8 weeks after the last dose of the study drug If sexually active, have used, and agree to use, highly effective contraception without interruption, for at least 28 days prior to starting investigational product, during the study (including dose interruptions), and for 17 weeks (119 days) after discontinuation of study treatment Refrain from breastfeeding a child or donating blood, eggs, or ovum for the duration of the study and for at least 17 weeks (119 days) after the last dose of study treatment  Male participants must:  Agree to use a condom, defined as a male latex condom or nonlatex condom NOT made out of natural (animal) membrane (e.g., polyurethane), during sexual contact with a pregnant female or a female of childbearing potential while participating in the study, during dose interruptions, and for at least 17 weeks (119 days) following investigational product discontinuation, even if he has undergone a successful vasectomy Refrain from donating blood or sperm for the duration of the study and for 17 weeks (119 days) after the last dose of study treatment Must agree to not participate in any other study of investigational drugs/devices while enrolled in this study  Exclusion Criteria:  Participant with SSc-pulmonary arterial hypertension (PAH) (except those participants with mild PAH on up to 2 oral drugs and mean pulmonary arterial pressure < 30 mmHg or low risk by risk calculator) Participant with airway obstruction (pre-bronchodilator forced expiratory volume in the first second/FVC ˂ 0.7) In the opinion of the investigator, other clinically significant pulmonary abnormalities (such as obstructive lung disease, asthma, etc.) Other investigational therapy received within 1 month or 6 half-lives (whichever is greater) prior to the Screening Visit Prior exposure to ACE-1334 or other TGF-β antibodies or any TGF-β family targeted biologic or hypersensitivity to the components of ACE-1334 Hypersensitivity to placebo or any of its components (Phase 2 only) Previous hematopoietic stem cell transplantation (HSCT) or HSCT planned within the next year Major surgical procedures planned during the study period Oral prednisone or equivalent > 10 mg/day Participant with history of gastric antral vascular ectasia or gastrointestinal bleed On anticoagulation therapy (such as prophylaxis anticoagulation, warfarin, direct thrombin inhibitors or other including low molecular weight subcutaneous or intravenous therapeutic heparin), or antiplatelet therapy other than daily aspirin for cardiovascular protection. Use of fish oil supplements within 2 weeks prior to randomization. History of any other medical condition that might interfere with a participant's ability to participate in the study Active clinically significant viral, bacterial, or fungal infection, or any episode of infection requiring hospitalization within 4 weeks prior to screening. Use of cyclophosphamide ≤ 6 months from screening Use of nintedanib or pirfenidone ≤ 28 days from screening Recent scleroderma renal crisis < 6 months before screening Use of tocilizumab ≤ 2 months from screening Hemoglobin < 10 g/dL at screening