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

Sii is the leading technology consulting, digital transformation, engineering, and business services vendor in Poland. We already employ more than 7 300 professionals and implement projects in a variety of industries for clients from many countries around the world. The Great Place to Work title, won 10 times in a row, proves that at Sii, we create a friendly work environment.

Sii ensures that all employment-related decisions are made solely on the basis of qualifications and competencies. We are committed to equal treatment of all individuals, regardless of any protected characteristics, and promoting a diverse and inclusive work environment.

• 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 Polish required
• Residing in Poland required

• 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

• Fluent Polish required
• Residing in Poland required