Senior Software Engineer, Payment Interfaces

Square, a Block company, builds integrated commerce and financial services products that help sellers manage payments, sales, and business operations. The role designs, builds, and scales customer-facing payment APIs and critical systems, while leading complex engineering projects and maintaining reliable, secure, high-volume production services.

Responsibilities:

• Design, build, maintain, and scale customer-facing payment APIs with clear contracts, predictable behavior, and compatibility that developers can rely on
• Lead complex projects end to end: clarify requirements, evaluate trade-offs, plan incremental delivery, and own testing, rollout, and ongoing maintenance
• Operate critical payment systems serving worldwide traffic. Participate in the team’s on-call rotation, investigate incidents, and follow through on fixes that improve reliability
• Improve performance, observability, fault tolerance, and security. Use automation to reduce repetitive operational work and make systems safer to change
• Work with product and engineering partners across Block to turn payment requirements into reusable capabilities
• Use AI-assisted tools for design, implementation, testing, and debugging. Critically evaluate their output and help teammates adopt approaches that improve quality and delivery
• Shape technical decisions within the team and develop others through design reviews, code reviews, documentation, and mentoring

Requirements:

• Experience building, maintaining, and scaling customer-facing financial or payment APIs, together with a strong understanding of payments. This is a requirement for the role
• Experience designing and operating high-volume, highly available distributed systems where correctness, security, and reliability matter
• An understanding of payment API concerns such as idempotency, retries, asynchronous processing, backward compatibility, and handling partial failures without compromising financial correctness
• A record of leading complex engineering projects from unclear requirements through production, making technical trade-offs, and coordinating work with partner teams
• Experience diagnosing production issues and improving systems through monitoring, incident response, and preventive engineering
• Fluency with AI-assisted engineering tools and the judgment to validate generated code, protect sensitive data, and recognize when deeper investigation is needed
• Clear written communication and experience helping other engineers develop their technical skills
• A minimum of 8 years of related experience with a Bachelor's degree; or 6 years and a Master's degree; or a PhD with 3 years experience; or equivalent experience