<PAGE>

                                                                   Exhibit 4.15

                          FIRST SUPPLEMENTAL INDENTURE

    FIRST SUPPLEMENTAL INDENTURE, dated as of June 19, 2000 (this "Supplemental
Indenture"), among Viatel, Inc., a Delaware corporation ("Viatel"), and The Bank
of New York, a New York banking corporation, as Trustee (the "Trustee").

                              W I T N E S S E T H:

    WHEREAS, the parties hereto are also the parties to that certain indenture,
dated as of April 12, 2000 (the "Indenture"), relating to the 7 3/4% Convertible
Junior Subordinated Debentures due April 15, 2015 issued by Viatel (the
"Convertible Debentures"); and

    WHEREAS, in accordance with Section 8.1 of the Indenture, the parties hereto
have agreed to amend, and the Board of Directors of Viatel has deemed it
necessary and desirable to amend, certain terms of the Indenture as described
below; and

    WHEREAS, all things necessary to make this Supplemental Indenture a valid
supplement to the Indenture according to the terms of the Indenture have been
done;

    NOW, THEREFORE, the parties hereto agree as follows:

    SECTION 1.  CERTAIN TERMS DEFINED IN THE INDENTURE.  All capitalized terms
used herein without definition herein shall have the meanings ascribed thereto
in the Indenture.

    SECTION 2.  AMENDMENTS TO THE INDENTURE.  The Indenture is hereby amended as
follows:

    (a) The first sentence of Section 12.1 of the Indenture is hereby amended by
       deleting the words "beginning July 11, 2000 through" appearing in the
       third line thereof and replacing such language with the following
       language:

       "prior to";

    (b) The first sentence of the fourteenth paragraph under the heading "Form
       of Reverse of Debenture" attached as Exhibit A to the Indenture is hereby
       amended by deleting the language reading "beginning July 11, 2000
       through" from the second line thereof and replacing such language with
       the following language:

       "prior to".

    SECTION 3.  GOVERNING LAW.  This Supplemental Indenture shall be governed
by, and construed in accordance with, the laws of the State of New York but
without giving effect to applicable principles of conflicts of law to the extent
that the application of the laws of another jurisdiction would be required
thereby.

    SECTION 4.  COUNTERPARTS.  This Supplemental Indenture may be signed in any
number of counterparts, each of which shall be an original, with the same effect
as if the signatures thereto and hereto were upon the same instrument.

    SECTION 5.  RATIFICATION.  Except as expressly amended hereby, each
provision of the Indenture shall remain in full force and effect and, as amended
hereby, the Indenture is in all respects ratified and confirmed by each of
Viatel and the Trustee.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

<PAGE>

    IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture to be duly executed as of the date first above written.

                          VIATEL, INC.

                          By:            /s/ JAMES P. PRENETTA
                               -------------------------------------------------
                                Name: James P. Prenetta
                                Title: Senior Vice President and General Counsel

                          THE BANK OF NEW YORK, as Trustee

                          By:               /s/ MING SHIANG
                               -----------------------------------------
                               Name: Ming Shiang
                               Title: Vice President


                                       2