Company: PAYX
Filing Date: 2025-03-31
Form Type: S-3ASR
Source: 0001193125-25-067979
Chunk: 1

Company: PAYCHEX INC
Filing Date: 2025-03-31
Form: S-3ASR
Chunk 1
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 Securities Act, check the following box. ☒ If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box. ☐ Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-acceleratedfiler, smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging growth company” in Rule 12b-2of the Exchange Act.

| Large accelerated filer |     | ☒                                               |     | Accelerated filer         |     | ☐ |
| Non-accelerated filer   |     | ☐ (Do not check if a smaller reporting company) |     | Smaller reporting company |     | ☐ |
| Emerging growth company |     | ☐                                               |     |                           |     |   |

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a) (2)(B) of the Securities Act. ☐

PROSPECTUS

Paychex, Inc.

Debt Securities

We may offer debt securities from time to time. Specific terms of these securities will be provided in supplements to this prospectus. You
should read this prospectus and any supplement carefully before you invest.

Investing in our securities involves risks. See “ Risk Factors” on page 5 of this prospectus and any risk factors described in any applicable prospectus supplement, as well as those risk factors in “Part I – Item 1A. Risk Factors” of our Annual Report on Form 10-Kfor the year ended May 31, 2024, as filed with the Securities and Exchange Commission, which is incorporated by reference herein, as well as in any other recently filed quarterly or current reports.

Neither the
Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or passed upon the adequacy or accuracy of this prospectus or any applicable prospectus supplement. Any representation to the
contrary is a criminal offense.

The date of this prospectus is March 31, 2025.

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