CONVERTIBLE SENIOR NOTES

Exhibit 4.1

UNLESS (i) THIS CERTIFICATE IS PRESENTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY TO THE ISSUER OR ITS AGENT FOR REGISTRATION OF TRANSFER, EXCHANGE OR PAYMENT, AND (ii) ANY CERTIFICATE ISSUED IS REGISTERED IN THE NAME OF CEDE & CO. OR IN SUCH OTHER NAME AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY (AND ANY PAYMENT HEREON IS MADE TO CEDE & CO. OR TO SUCH OTHER ENTITY AS IS REQUESTED BY AN AUTHORIZED REPRESENTATIVE OF THE DEPOSITORY TRUST COMPANY), ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL SINCE THE REGISTERED OWNER HEREOF, CEDE & CO., HAS AN INTEREST HEREIN.

TRANSFERS OF THIS GLOBAL SECURITY SHALL BE LIMITED TO TRANSFERS IN WHOLE, BUT NOT IN PART, TO NOMINEES OF THE DEPOSITORY TRUST COMPANY OR TO A SUCCESSOR THEREOF OR SUCH SUCCESSOR’S NOMINEE.

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Washington Real Estate Investment Trust

3  ⁷ / 8 % Convertible Senior Note due September 15, 2026

Washington Real Estate Investment Trust, a real estate investment trust organized under the laws of the State of Maryland, promises to pay to Cede & Co. or registered assigns, the Principal Amount of Fifteen Million Dollars ($15,000,000) on September 15, 2026.

This Note shall bear cash interest at the rate of 3.875% per annum. This Note is convertible as specified on the other side of this Note.

Additional provisions of this Note are set forth on the other side of this Note.

Dated: January 30, 2007

TRUSTEE’S CERTIFICATE OF AUTHENTICATION

This is one of the Notes of the series designated “3  ⁷ / 8 % Convertible Senior Notes due September 15, 2026” pursuant to the within-mentioned Indenture.

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(REVERSE SIDE OF SECURITY)

Washington Real Estate Investment Trust

3  ⁷ / 8 % Convertible Senior Note due September 15, 2026

1. Interest

This Note shall bear cash interest at the rate of 3.875% per annum. Interest on this Note shall accrue from January 22, 2007, or from the most recent date to which interest has been paid or provided for. Interest shall be payable semiannually in arrears on March 15 and September 15 of each year, beginning on March 15, 2007, to the Holders of record of Notes at the close of business on the March 1 or September 1 immediately preceding such Interest Payment Date; provided, however, that interest payable upon redemption or purchase of the Notes by the Trust shall be paid to the Person to whom Principal is payable, unless the Redemption Date, Repurchase Date or Fundamental Change Purchase Date, as the case may be, is an Interest Payment Date. Each payment of cash interest on this Note shall include interest accrued for the period commencing on and including the immediately preceding Interest Payment Date (or, if none, January 22, 2007) through the day before the applicable Interest Payment Date. Any payment required to be made on any day that is not a Business Day shall be made on the next succeeding Business Day. Interest shall be calculated using a 360-day year composed of twelve 30-day months. Interest shall cease to accrue on this Note upon conversion, redemption, repurchase or on the Maturity Date.

If any portion of the Principal Amount hereof or any premium hereon is not paid when due or if interest due hereon or any portion of such interest is not paid when due, then in each such case the overdue amount shall, to the extent permitted by law, bear interest at the rate of 3.875% per annum, compounded semiannually, which interest shall accrue from the date such overdue amount was originally due to the date payment of such amount, including interest thereon, has been made or duly provided for. All such interest shall be payable on demand.

2. Method of Payment

Subject to the terms and conditions of the Indenture, the Trust will make payments in respect of Redemption Prices, Repurchase Prices and Fundamental Change Purchase Prices and at the Maturity Date to Holders who surrender Notes to a Paying Agent to collect such payments in respect of the Notes. The Trust will pay any cash amounts in money of the United States that at the time of payment is legal tender for payment of public and private debts. The Trust may pay interest thereon by check mailed to the address of the Holder as it appears on the register of the Notes; provided, however, that all payments on any Global Security shall be made by wire transfer of immediately available funds to the account of the holder of such Global Security.

3. Paying Agent, Conversion Agent And Registrar

Initially, The Bank of New York Trust Company, N.A. (the “Trustee”), will act as Paying Agent, Conversion Agent and Registrar. The Trust may appoint and change any Paying Agent, Conversion Agent or Registrar without notice, other than notice to the Trustee. The Trust or any of its Subsidiaries or any of their Affiliates may act as Paying Agent, Conversion Agent or

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Registrar. The Trust may maintain deposit accounts and conduct other banking transactions with the Trustee in the normal course of business.

4. Indenture

The Trust issued the Notes under an Indenture dated as of August 1, 1996, between the Trust and the Trustee. The terms of the Notes include those stated in the Indenture, those made part of the Indenture by reference to the Trust Indenture Act of 1939, as in effect from time to time (the “TIA”) and those made part of the Indenture through Section 301 of the Indenture. Capitalized terms used herein and not defined herein have the meanings ascribed thereto in such Indenture. The Notes are subject to all such terms, and Holders are referred to the Indenture for a statement of those terms.

The Notes are unsecured senior obligations of the Trust and are initially limited to up to $150,000,000 aggregate Principal Amount. Additional Notes may be offered in the future on the same terms and conditions, except for any difference in the issue price and interest accrued prior to the issue date of the additional Notes, and with the same CUSIP number as the Notes so long as such additional Notes are fungible for U.S. federal income tax purposes with the Notes. Any additional Notes will rank equally and ratably with the Notes and will be treated as a single series of debt securities for all purposes under the Indenture.

5. Optional Redemption by the Trust

On or after September 20, 2011, the Trust may redeem the Notes, in whole or in part, for cash at a Redemption Price equal to 100% of the Principal Amount to be redeemed plus accrued and unpaid interest, if any, to, but excluding, the Redemption Date.

6. Purchase by the Trust at the Option of the Holder

Notes or portions thereof shall be purchased by the Trust at the option of the Holder for cash on the Repurchase Dates of September 15, 2011, September 15, 2016 and September 15, 2021, at a Repurchase Price equal to 100% of the principal amount of the Notes to be repurchased, plus accrued and unpaid interest, if any, to, but excluding, the Repurchase Date.

Notes in denominations larger than $1,000 of Principal Amount may be purchased in part, but only in integral multiples of $1,000 of Principal Amount.

7. Purchase by the Trust at the Option of th