KSeF (Krajowy System e-Faktur, the National e-Invoice System) is the Polish government's central system for structured invoices: invoices in a fixed machine-readable format rather than PDF or paper. It is run by the Ministry of Finance (Ministerstwo Finansów) and serves to issue, send, receive and store these invoices. A seller issues an invoice into KSeF, and the buyer receives it from KSeF.
Use became mandatory in stages: from 1 February 2026 for companies whose 2024 sales exceeded 200 million złoty including VAT, and from 1 April 2026 for all others. Until 31 December 2026, invoices outside KSeF are still allowed when their monthly total stays at or below 10,000 złoty. A business connects to KSeF in several ways, among them: accounting software integrated through the KSeF API; the free web application Aplikacja Podatnika KSeF, which also manages user permissions and tokens and downloads the official receipt (UPO); or the free mobile app for issuing and receiving invoices on a smartphone.
It is a public system, free to use: the Ministry of Finance states that no paid software is needed, because it provides the web and mobile apps free of charge.