What is KSeF and why is it mandatory?
KSeF is Poland's national structured e-invoice system, managed by the Polish Ministry of Finance. Unlike Peppol, KSeF is a centralized system: all invoices go through the government's central registry, which validates them, assigns a unique identifier (KSeF ID), and archives them. Only invoices with a KSeF ID have fiscal validity in Poland.
Feb 1, 2026
Date for large companies
Apr 1, 2026
Date for other taxpayers
KSeF API
Centralized system
KSeF implementation timeline
KSeF technical requirements
Format: FA(3) — national structured XML
Invoices must follow the FA(3) schema defined by the Polish Ministry of Finance. It is a proprietary XML format, based on EN16931 principles but with national structure.
Mandatory KSeF ID
Each invoice receives a unique KSeF number, assigned automatically by the system. From January 1, 2027 it must be quoted in payment transfers between active VAT taxpayers.
FAQ — KSeF Poland
What is the KSeF ID and why is it important?
The KSeF number is the unique identifier that KSeF assigns automatically to every invoice issued in the system. From January 1, 2027, active VAT taxpayers must also quote it in payment transfers between them.
Does KSeF affect foreign companies selling in Poland?
The obligation to issue invoices in KSeF applies to taxpayers established in Poland. Taxpayers with no registered seat or fixed establishment in Poland are excluded from that obligation, although they may issue invoices in KSeF voluntarily. Holding a Polish VAT number does not change this.
What happens if I send an invoice without using KSeF after the mandate?
Invoices issued outside the KSeF system after the mandate dates have no legal validity in Poland. The recipient can reject them and the issuer may face sanctions from the Polish tax administration (KAS).