The purpose of this study is to test DS-7080a, a monoclonal antibody, as a new treatment for neovascular age-related macular degeneration (AMD) and diabetic macular edema (DME). The hypothesis of the study is that DS-7080a is safe and shows preliminary efficacy in patients with these conditions either alone or in combination with ranibizumab. This study is organized into 3 Parts: Part 1 Dose Escalation in AMD participants, Part 2 Dose Expansion in AMD participants, and Part 3 Dose Expansion in DME participants. In Part 1, participants will be enrolled into 3 sequential, ascending dose-level cohorts in non-randomized uncontrolled manner with the main purpose to determine the recommended dose. In Part 2, participants will be randomized to 1 of 3 arms of either monotherapy with DS-7080a or monotherapy with ranibizumab, which is an active control, or combination therapy of DS-7080a plus ranibizumab (ranibizumab will be administered 30 minutes prior to DS-7080a). In Part 3, subjects with DME will be assigned to 1 of 2 arms of either monotherapy with DS-7080a or monotherapy with ranibizumab. DS-7080a or ranibizumab will be administered 3 times: on Baseline/Day 1, Day 29, and Day 57. Both Parts 2 and 3 will consist of 8 visits including a 14-day screening phase, an 84-day treatment period, and a 28-day follow-up period.