Kotlin Developer (f/m/x)

Sii Poland; Białystok; On-site; PLN 180,000 - 280,000; Full time

Benefits offered by this job

Great Place to Work

Profit sharing

Medical care

Internal trainings

Job summary

Sii Poland in Białystok is seeking a senior backend engineer to design and maintain scalable microservices in Kotlin, focusing on a fintech platform that streamlines expense, payment, and invoice workflows.

You will collaborate with cross-functional teams across Platform, Services, SMB, and other domains, optimize performance, and lead code reviews while mentoring peers in a Polish fintech environment.

Qualifications

• 5+ years of professional software development experience
• Expertise in Kotlin and distributed systems
• Experience with microservices and cloud environments (AWS, Kubernetes)
• Relational databases (PostgreSQL) and testing/observability tools (JUnit, Testcontainers, Grafana)
• Strong problem-solving and mentorship mindset

Responsibilities

• Lead design, build, and maintenance of scalable Kotlin microservices
• Collaborate with cross-functional teams across Platform, Services, SMB, and more
• Analyze performance and implement optimizations for reliability and scalability
• Participate in code reviews, post-mortems, and mentor engineers
• Improve architecture and guide the team through technical challenges and migrations

Skills

Kotlin

Distributed systems

Microservices

AWS

Kubernetes

PostgreSQL

JUnit

Testcontainers

Grafana

English B2

Mentorship

Job description

Join a team working on a modern fintech platform that supports company expense management. The project focuses on developing a finance solution that enables seamless integration of financial tools (including payment cards, expense management, and invoice handling) directly into partner systems such as banks, accounting platforms, and HR solutions. The product being developed allows companies to simplify financial processes, automate settlements, and gain full real-time control over their spending.

Your tasks

• Leading initiatives to design, build, and maintain scalable microservices primarily using Kotlin
• Collaborating with cross-functional teams to develop innovative solutions across Platform, Services, SMB, and other domains
• Analyzing system performance and implementing optimizations to ensure high reliability and scalability
• Participating in code reviews, post-mortems, and providing mentorship to other engineers
• Proactively improving the architecture and quality of solutions, and leading the team through technical challenges and migrations

Requirements

• Minimum 5 years of professional experience in software development
• Expertise in server-side languages, especially Kotlin, and experience with distributed systems, microservices, and cloud environments (e.g., AWS, Kubernetes)
• Proficiency with relational databases like PostgreSQL, testing frameworks (e.g., JUnit, Testcontainers), and observability tools like Grafana
• Strong problem-solving skills and a collaborative mindset to mentor and upskill your teammates
• Very good knowledge of English - min. B2 level
• Residing in Poland required

Nice to have

• Previous work in the fintech industry or with financial products
• Knowledge of topics related to payments, cards, or accounting
• Experience working in a scale-up environment
• Proficiency in one of the following front-end technologies: React or TypeScript

Job no. JOB-EQKGG

Sii ensures that all hiring decisions are made solely on the basis of qualifications and competence. We are committed to equal and fair treatment of all, regardless of legally protected characteristics. At Sii, we promote a diverse and inclusive work environment, in full compliance with applicable anti-discrimination laws.

Benefits For You

• Great Place to Work
• Solid financial situation
• Contracts with the biggest brands
• Centre of internal trainings
• Many experts you can learn from
• Open and accessible management team
• Profit sharing
• Passion Sponsorship program
• Regular integration events and trips
• Comfortable and well-equipped offices
• MySii app
• Medical care