diff --git a/ceidg_api_service.py b/ceidg_api_service.py
new file mode 100644
index 0000000..a7136cf
--- /dev/null
+++ b/ceidg_api_service.py
@@ -0,0 +1,156 @@
+#!/usr/bin/env python3
+"""
+CEIDG API Service
+==================
+
+Service module for fetching company data from CEIDG (Centralna Ewidencja
+i Informacja o Działalności Gospodarczej) using the official API at
+dane.biznes.gov.pl.
+
+Provides fetch_ceidg_by_nip function for membership application workflow.
+"""
+
+import os
+import logging
+import requests
+from typing import Optional, Dict, Any
+from dotenv import load_dotenv
+
+# Load environment variables
+load_dotenv()
+
+logger = logging.getLogger(__name__)
+
+# API Configuration
+CEIDG_API_V3_URL = "https://dane.biznes.gov.pl/api/ceidg/v3/firmy"
+CEIDG_API_KEY = os.getenv("CEIDG_API_KEY")
+CEIDG_TIMEOUT = 15  # seconds
+
+
+def fetch_ceidg_by_nip(nip: str) -> Optional[Dict[str, Any]]:
+    """
+    Fetch company data from CEIDG API by NIP.
+
+    Args:
+        nip: NIP number (10 digits, no dashes)
+
+    Returns:
+        Dictionary with company data or None if not found
+    """
+    if not CEIDG_API_KEY:
+        logger.warning("CEIDG_API_KEY not configured - CEIDG lookup disabled")
+        return None
+
+    # Clean NIP
+    nip = nip.strip().replace('-', '').replace(' ', '')
+    if not nip or len(nip) != 10 or not nip.isdigit():
+        logger.warning(f"Invalid NIP format: {nip}")
+        return None
+
+    headers = {
+        "Authorization": f"Bearer {CEIDG_API_KEY}",
+        "Accept": "application/json"
+    }
+
+    try:
+        logger.info(f"Fetching CEIDG data for NIP {nip}")
+
+        response = requests.get(
+            CEIDG_API_V3_URL,
+            params={"nip": nip},
+            headers=headers,
+            timeout=CEIDG_TIMEOUT
+        )
+
+        if response.status_code == 401:
+            logger.error("CEIDG API authentication failed - check CEIDG_API_KEY")
+            return None
+
+        if response.status_code == 404:
+            logger.info(f"NIP {nip} not found in CEIDG")
+            return None
+
+        if response.status_code != 200:
+            logger.error(f"CEIDG API error: {response.status_code} - {response.text[:200]}")
+            return None
+
+        data = response.json()
+
+        # Handle response format - can be list or dict
+        if isinstance(data, list):
+            if not data:
+                logger.info(f"NIP {nip} not found in CEIDG (empty list)")
+                return None
+            firma = data[0]
+        elif isinstance(data, dict):
+            if 'firmy' in data:
+                firmy = data.get('firmy', [])
+                if not firmy:
+                    logger.info(f"NIP {nip} not found in CEIDG")
+                    return None
+                firma = firmy[0]
+            else:
+                firma = data
+        else:
+            logger.error(f"Unexpected CEIDG response format: {type(data)}")
+            return None
+
+        # Extract address
+        adres = firma.get('adresDzialalnosci', {}) or firma.get('adres', {}) or {}
+        if isinstance(adres, str):
+            adres = {'full': adres}
+
+        # Build normalized result
+        result = {
+            'firma': firma.get('nazwa') or firma.get('nazwaSkrocona'),
+            'nip': firma.get('nip'),
+            'regon': firma.get('regon'),
+            'adresDzialalnosci': {
+                'kodPocztowy': adres.get('kodPocztowy') or adres.get('kod'),
+                'miejscowosc': adres.get('miejscowosc') or adres.get('miasto'),
+                'ulica': adres.get('ulica'),
+                'budynek': adres.get('budynek') or adres.get('nrDomu') or adres.get('nrBudynku'),
+                'lokal': adres.get('lokal') or adres.get('nrLokalu'),
+            },
+            'email': firma.get('email') or firma.get('adresEmail'),
+            'stronaWWW': firma.get('stronaWWW') or firma.get('www') or firma.get('strona'),
+            'telefon': firma.get('telefon'),
+            'dataRozpoczeciaDzialalnosci': firma.get('dataRozpoczeciaDzialalnosci') or firma.get('dataWpisuDoCeidg'),
+            'status': firma.get('status'),
+            'raw': firma
+        }
+
+        logger.info(f"CEIDG data found for NIP {nip}: {result['firma']}")
+        return result
+
+    except requests.exceptions.Timeout:
+        logger.error(f"CEIDG API timeout for NIP {nip}")
+        return None
+    except requests.exceptions.RequestException as e:
+        logger.error(f"CEIDG API request error for NIP {nip}: {e}")
+        return None
+    except Exception as e:
+        logger.error(f"Error fetching CEIDG data for NIP {nip}: {e}")
+        return None
+
+
+# For testing
+if __name__ == '__main__':
+    import sys
+    import json
+
+    if len(sys.argv) < 2:
+        print("Usage: python ceidg_api_service.py <NIP>")
+        print("Example: python ceidg_api_service.py 5881571773")
+        sys.exit(1)
+
+    nip = sys.argv[1]
+    print(f"Pobieranie danych z CEIDG API dla NIP: {nip}")
+    print("=" * 60)
+
+    data = fetch_ceidg_by_nip(nip)
+
+    if data:
+        print(json.dumps(data, indent=2, ensure_ascii=False, default=str))
+    else:
+        print(f"Nie znaleziono firmy o NIP {nip} w CEIDG")
diff --git a/krs_api_service.py b/krs_api_service.py
index 4cc8266..b9b814c 100644
--- a/krs_api_service.py
+++ b/krs_api_service.py
@@ -440,6 +440,121 @@ def format_address(krs_data: KRSCompanyData) -> str:
     return ', '.join(parts)
 
 
+# ============================================================
+# KRS API Service Class (for search_by_nip compatibility)
+# ============================================================
+
+class KRSApiService:
+    """
+    KRS API Service class providing unified interface for KRS lookups.
+
+    Note: KRS Open API doesn't support direct NIP lookup.
+    This class uses rejestr.io unofficial API as a fallback.
+    """
+
+    REJESTR_IO_URL = "https://rejestr.io/api/v2/org"
+    REJESTR_IO_TIMEOUT = 10
+
+    def search_by_nip(self, nip: str) -> Optional[Dict[str, Any]]:
+        """
+        Search KRS by NIP number.
+
+        Since KRS Open API doesn't support NIP lookup, this method:
+        1. First tries rejestr.io API (unofficial but reliable)
+        2. Falls back to checking our database for KRS number
+        3. Then fetches full data from KRS Open API
+
+        Args:
+            nip: NIP number (10 digits)
+
+        Returns:
+            Dictionary with company data or None if not found
+        """
+        import logging
+        logger = logging.getLogger(__name__)
+
+        # Clean NIP
+        nip = nip.strip().replace('-', '').replace(' ', '')
+        if not nip or len(nip) != 10 or not nip.isdigit():
+            return None
+
+        # Try rejestr.io first (supports NIP lookup)
+        krs_number = self._get_krs_from_rejestr_io(nip)
+
+        if not krs_number:
+            # Try our database
+            krs_number = self._get_krs_from_database(nip)
+
+        if not krs_number:
+            logger.info(f"No KRS found for NIP {nip}")
+            return None
+
+        # Fetch full data from KRS Open API
+        logger.info(f"Found KRS {krs_number} for NIP {nip}, fetching details")
+        krs_data = get_company_from_krs(krs_number)
+
+        if not krs_data:
+            return None
+
+        # Return as dict with expected format
+        return krs_data.to_dict()
+
+    def _get_krs_from_rejestr_io(self, nip: str) -> Optional[str]:
+        """Try to get KRS number from rejestr.io API."""
+        import logging
+        logger = logging.getLogger(__name__)
+
+        try:
+            # rejestr.io API endpoint for NIP lookup
+            url = f"{self.REJESTR_IO_URL}"
+            params = {"nip": nip}
+
+            response = requests.get(url, params=params, timeout=self.REJESTR_IO_TIMEOUT)
+
+            if response.status_code == 200:
+                data = response.json()
+                # Handle different response formats
+                if isinstance(data, list) and data:
+                    krs = data[0].get('krs')
+                    if krs:
+                        return str(krs).zfill(10)
+                elif isinstance(data, dict):
+                    krs = data.get('krs')
+                    if krs:
+                        return str(krs).zfill(10)
+
+            logger.debug(f"rejestr.io: No KRS found for NIP {nip}")
+            return None
+
+        except Exception as e:
+            logger.debug(f"rejestr.io lookup failed: {e}")
+            return None
+
+    def _get_krs_from_database(self, nip: str) -> Optional[str]:
+        """Check our database for KRS number."""
+        import logging
+        logger = logging.getLogger(__name__)
+
+        try:
+            from database import SessionLocal, Company
+            db = SessionLocal()
+            try:
+                company = db.query(Company).filter(Company.nip == nip).first()
+                if company and company.krs:
+                    logger.debug(f"Found KRS {company.krs} in database for NIP {nip}")
+                    return company.krs
+            finally:
+                db.close()
+        except Exception as e:
+            logger.debug(f"Database KRS lookup failed: {e}")
+
+        return None
+
+
+# Singleton instance for import
+krs_api_service = KRSApiService()
+
+
 # CLI for testing
 if __name__ == '__main__':
     import sys