Senior Fullstack .NET Developer (f/m/x)

We are looking for a highly experienced Fullstack .NET Developer with strong React skills to join a long‑term project delivered by Sii for an international organization recognized for its advanced laboratory and diagnostics technologies. You will work on the development and modernization of complex systems, including legacy monolithic applications and WPF components, while contributing to new features built on ASP.NET Core and modern JavaScript frameworks.

The role is hybrid, with 2–3 days per week in our Wrocław office.

If you're ready for your next big challenge — apply now!

Your tasks

• Design, develop, and maintain fullstack applications using ASP.NET Core, C#, and React
• Modernize and refactor existing monolithic systems, ensuring performance and scalability
• Implement and enhance UI components, including working with WPF where necessary
• Collaborate with cross‑functional teams on architecture, code reviews, and technical direction
• Troubleshoot, optimize, and ensure high-quality, testable, and maintainable code

Requirements

• At least 6 years of experience as a .NET/Fullstack Developer
• Strong hands-on experience with C#, ASP.NET Core, and React/JavaScript
• Knowledge of monolithic architectures and refactoring approaches
• Experience with WPF or willingness to maintain existing desktop components
• Very good English (B2+) for daily communication in an international environment
• Fluent in Polish required
• Residing in Poland required

Job no. JOB-PMTE8

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