6-K

QuickLinks -- Click here to rapidly navigate through this document

FORM 6-K

SECURITIES AND EXCHANGE COMMISSION
Washington D.C. 20549

Report of Foreign Issuer

Pursuant to Rule 13a-16 or 15d-16 of
the Securities Exchange Act of 1934

For the month of October 2004 Commission File No.: 1-31349

THE THOMSON CORPORATION
(Translation of registrant's name into English)

Metro Center, One Station Place
Stamford, Connecticut 06902, United States
(Address of principal executive offices)

Indicate by check mark whether the registrant files or will file annual reports under cover Form 20-F or Form 40-F.

Form 20-F o                        Form 40-F ý

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(1):                             

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(7):                             

Indicate by check mark whether by furnishing the information contained in this Form, the registrant is also thereby furnishing the information to the Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934.

Yes o                        No ý

If "Yes" is marked, indicate below the file number assigned to the registrant in connection with Rule 12g3-2(b): 82-                             .

EXHIBIT INDEX

Exhibit Number	Description
99.1	Press release dated October 18, 2004

SIGNATURES

        Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

THE THOMSON CORPORATION
By:	/s/  DEIRDRE STANLEY Name: Deirdre Stanley Title: Senior Vice President and General Counsel

Date: October 18, 2004

QuickLinks

EXHIBIT INDEX
SIGNATURES

EX-99.1

The Thomson Corporation Metro Center, One Station Place Stamford, CT 06902 Tel (203) 539-8000 www.thomson.com
News Release
Media Contacts: Jason Stewart Vice President, Public Relations (203) 539-8339 jason.stewart@thomson.com FOR IMMEDIATE RELEASE	Investor Contact: Frank J. Golden Vice President, Investor Relations (203) 539-8470 frank.golden@thomson.com

THOMSON FINANCIAL NOTIFIED OF SEC INQUIRY INTO
ITS CAPITAL MARKETS INTELLIGENCE SERVICE

STAMFORD, Conn., October 18, 2004 — Thomson Financial today announced that it has received a subpoena from the Securities and Exchange Commission (SEC) for certain documents related to the operations of its Capital Markets Intelligence (CMI) business. Thomson Financial is cooperating fully with the SEC and has been informed that the inquiry should not be construed as an indication by the Commission or its staff that any violation of law has occurred.

        Thomson Financial is one of several companies providing market intelligence services. Thomson Financial's CMI business collects stock ownership data solely as an appointed agent of its public company clients seeking a better understanding of their institutional shareholder base. In 2003, approximately $33 million of Thomson Financial's CMI revenues were related to the identification of institutional investors for its clients.

        Thomson Financial is a $1.5 billion provider of information and technology solutions to the worldwide financial community and part of The Thomson Corporation (NYSE: TOC; TSX: TOC). With revenues of $7.44 billion, The Thomson Corporation is a leading global provider of integrated information solutions to more than 20 million users in approximately 130 countries.