Senior Backend Engineer - (Kotlin/JVM)

SumUp builds simple, affordable financial tools that help over 4 million small businesses across 37 markets get paid, manage their money, and grow. Beyond card payments, we offer a full suite of financial products - business banking, lending, invoicing, and money transfers - giving merchants everything they need in one place. We're a global fintech with over 3,000 people across 20+ offices, and Sofia has been one of our most important engineering hubs since the very beginning. Our Transfers Excellence squad sits within SumUp's Global Bank, and owns the core services that power how merchants and consumers move money. E.g. IBAN-based transfers, money in and out, and peer-to-peer payments via the SumUp consumer app. With the high volume transfers per hour across our European markets. That means reliability isn't a nice-to-have - it's the baseline for us. Right now, the team is replacing a set of market-specific implementations with a single, unified global transfer platform that will serve Europe, the UK, Brazil, Chile, and eventually the US and Canada.  Join our team and contribute directly to platform-level architecture that will shape how SumUp moves money at scale - work that directly impacts millions of small businesses every day. Our tech stack includes Kotlin, Java, Go, a bit of Elixir, AWS, Kafka, PostgreSQL, and Kubernetes, supported by a strong observability toolchain with Prometheus, Grafana, and Honeycomb. We also actively use AI‑assisted development tools such as Cursor, GitHub Copilot, and others. 🎥 Get a feel for life in our Sofia office. WHAT YOU'LL DO * Build and maintain backend services that sit at the heart of SumUp's transfer infrastructure, taking end-to-end ownership from development through to observability, testing and production support * Contribute to the migration from market-specific services toward a unified global transfer platform, working across a mix of active services and legacy systems being decomposed * Raise well-scoped pull requests, conduct thorough code reviews, and help maintain high standards of code quality and test coverage across the team * Participate in incident reviews and the on-call rotation, taking accountability for the reliability of the services you build * Contribute to the technical roadmap by participating in architectural discussions and proposing solutions for complex, distributed systems challenges * Apply AI tooling to accelerate your own engineering workflows and contribute to raising the team's overall AI literacy YOU'LL BE GREAT FOR THIS ROLE IF… * Solid experience as a Backend Software Engineer in modern languages (e.g. Kotlin, Go, Java, etc.). Kotlin is the team's primary language and we also work with Java, Golang, and Elixir. * Solid understanding of distributed systems and event-driven architecture. Experience with async processing and messaging tools like Kafka (or similar). * Experience working with microservice architectures, including Spring Boot and event-driven patterns, alongside solid knowledge of relational databases and API design * Comfortable working in high-throughput, regulated environments where reliability matters. Defensive coding and attention to edge cases are part of the job * Clear communicator in written and spoken English, with a collaborative approach to working across global teams  * Curious about the product impact, not just the code. Comfortable thinking about how backend decisions affect the end user experience.  WHY YOU SHOULD JOIN SUMUP * 🌍 Opportunity to work with SumUppers globally on large-scale fintech products used by millions of businesses worldwide, from our modern Sofia office. This involves an office-first setup  * 🌈 Commitment to Diversity and Inclusion: be part of a workplace that values and promotes diversity, fostering an inclusive environment where everyone's perspectives are respected and embraced  * 🚀 Enrolment onto our Virtual Stock Option programme: you will own a stake in SumUp's future success  * 📚 A dedicated annual L&D budget of €2000 and 10 paid days of educational leave which can be used to attend conferences and/or advance your career through further education  * 🏖 Generous time off: 25 days of paid leave, plus one extra day per year of service (up to 35), along with public holidays and special leave options  * 🏥 Peace of mind: additional health insurance and life insurance  * 💪 Stay active: on-site yoga and a co-sponsored Multisport card offering access to hundreds of sports facilities  * 🍽 Meal support: benefit from a tax-free food voucher program plus flexibility via our Re:Benefits platform  * 🚌 Easy commute: free shuttle buses connecting directly to Joliot-Curie metro station  * 🌴 Break4me: 1-month sabbatical after 3 years of service  * 🔗 Referral Bonus: earn additional rewards by referring talented individuals to join the SumUp team ABOUT SUMUP Be empowered to do more that matters. At SumUp, we're on a mission to empower small businesses across the globe by providing simple and affordable tools that allow them to thrive. Today, over 4 million businesses in 37 markets rely on SumUp as their financial partner to manage payments, finance and customer relationships. Our commitment to small businesses is reflected in our diverse team of over 3,000 SumUppers from over 90 nationalities, united by global collaboration and an innovative mindset. Our core values lay the foundation for who we are and what we stand for, shaping our work culture and driving our success. We foster inclusivity and a continuous learning culture, providing a safe space for personal and professional growth. Our differences make us unique and strong as we strive to create an environment where everyone belongs and feels supported, no matter how they identify. SumUp is proud to be an Equal Employment Opportunity employer, actively seeking and embracing diversity in our workforce. We don't make hiring or employment decisions based on race, colour, religion or religious belief, ethnic or national origin, nationality, sex, gender, gender identity, sexual orientation, disability, age or any other basis protected by applicable laws or prohibited by company policy. Our commitment extends beyond recruitment to creating a safe and respectful workplace where harassment of any form is strictly prohibited. Discover more about our culture and opportunities on our careers website, and follow our journey on LinkedIn, Instagram, and TikTok. Job Application Tip We recognise that candidates feel they need to meet 100% of the job criteria in order to apply for a job. Please note that this is only a guide. If you don’t tick every box, it’s ok too because it means you have room to learn and develop your career at SumUp.

SumUp enables businesses to get paid easily, process orders quickly, sell online instantly and manage their money more efficiently. We create the tools businesses need to make their business and their customer experience thrive.  The payments processing platform, consisting of other sub-platforms, has powered SumUp’s global business for more than 10 years now. It’s developed in Sofia by the Payments Tribe, composed of several teams (squads). Each squad is cross-functional, autonomous and co-located, working on its part of the platform end-to-end.  We are scaling and improving existing products, while developing new ones - an exciting time to join. You would make an impact by improving and co-creating our existing and new microservices architecture platforms, solving scaling and new development challenges. Scalability and high availability are among our focus areas.  What you’ll do * Design and evolve systems that process millions of transactions reliably and at scale * Build and maintain backend services in a microservices architecture * Contribute to API design for internal services and external integrations * Write clean, maintainable, and scalable code with strong test coverage * Participate in architectural discussions and drive technical decisions * Collaborate closely with engineers, QA, and DevOps to deliver production-ready systems * Continuously improve engineering practices, including leveraging AI to iterate faster You will be great for this role if you have: * Strong programming experience in any modern language (e.g. Go, Java, Python, Kotlin, etc.). We care more about how you think and design systems than the specific language you use * Solid understanding of system design, scalability, and distributed systems * Experience building APIs and working with data storage systems * Ability to learn new technologies quickly and pragmatically * Strong problem-solving skills and a structured way of thinking * Ownership mindset and collaborative approach to teamwork * Interest in incorporating AI as part of your development workflow Nice to have: * Experience with cloud platforms (e.g. AWS) * Familiarity with containers and orchestration (Docker, Kubernetes) * Experience with event-driven systems and messaging (e.g. Kafka) If you don’t meet every requirement but are excited about the role, we’d still love to hear from you. Why you should join SumUp * Make an impact on a global scale - our systems serve millions of merchants * Learn from experienced engineers while working on complex, real-world problems * Be part of a tech-agnostic team that uses the right tools for the job * Collaborate with SumUppers worldwide on large-scale fintech products used by millions, based in our Sofia office (office-first setup). * Access a €2,000 annual L&D budget to attend conferences or advance your career through education. * Enjoy 25 days of paid leave (increasing with tenure), plus paid vacations for special occasions and a 1-month sabbatical every 3 years. * Benefit from extra perks like health and life insurance, co-sponsored Multisport cards, childbirth/adoption bonuses, discounts, a referral program, 100 EUR monthly food vouchers, 40 EUR flexible benefits, and more. About SumUp We believe in the everyday hero. Small business owners are at the heart of all we do, so we create powerful, easy-to-use financial solutions to help them run their businesses. With a founder’s mentality and a 'team-first’ attitude, our diverse teams across Europe, South America and the United States work together to ensure that the small business owners we partner with can be successful doing what they love.  SumUp is an Equal Employment Opportunity employer that proudly pursues and hires a diverse workforce. SumUp does not make hiring or employment decisions on the basis of race, color, religion or religious belief, ethnic or national origin, nationality, sex, gender, gender identity, sexual orientation, disability, age or any other basis protected by applicable laws or prohibited by company policy. SumUp also strives for a healthy and safe workplace and strictly prohibits harassment of any kind. SumUp will not accept unsolicited resumes from any source other than directly from a candidate.   Job Application Tip We recognise that candidates feel they need to meet 100% of the job criteria in order to apply for a job. Please note that this is only a guide. If you don’t tick every box, it’s ok too because it means you have room to learn and develop your career at SumUp.