Company: PAYX
Filing Date: 2025-04-09
Form Type: 424B2
Source: 0001193125-25-076822
Chunk: 8

Company: PAYCHEX INC
Filing Date: 2025-04-09
Form: 424B2
Chunk 8
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 our reputation and the value of our brand; |

| • |     | the risk that the Acquisition may not be completed in a timely manner or at all, which may adversely affect our 
 business and the price of our common stock;                                                                     |

| • |     | the occurrence of any event, change or other circumstance or condition that could give rise to the termination of 
 the Merger Agreement; and                                                                                         |

| • |     | potential litigation relating to the Acquisition that could be instituted against the parties to the Merger 
 Agreement or their respective directors or officers, including the effects of any outcomes related thereto. |

Any of these factors, as well as other factors discussed in Part I, Item 1A, “Risk Factors” of the Annual Report on Form 10-K (“Form 10-K”)incorporated herein by reference, and in our periodic filings with the Securities and Exchange Commission (the “SEC”), could cause our actual results to differ materially from our anticipated results. The information provided in this prospectus supplement is based upon the facts and circumstances known as of the date of this report, and any forward-looking statements made by us in this prospectus supplement speak only as of the date on which they are made. Except as required by law, we undertake no obligation to update these forward-looking statements after the date of filing this prospectus supplement with the SEC to reflect events or circumstances after such date, or to reflect the occurrence of unanticipated events. S-v

SUMMARY The following summary highlights information contained or incorporated by reference in this prospectus supplement and the accompanying prospectus. It does not contain all of the information that you should consider before investing in the notes. You should carefully read this entire prospectus supplement, as well as the accompanying prospectus and the documents incorporated by reference in this prospectus supplement and the accompanying prospectus and any applicable free writing prospectus, prior to making your investment decision. Paychex We are an industry-leading human capital management (“HCM”) company delivering a full suite of technology and advisory services in human resources (“HR”), employee benefit solutions, insurance and payroll processing for small- to medium-sizedbusinesses and their employees across the U.S. and parts of Europe. For any organization, a key function is effective HCM, which requires both resources and expertise. Organizations are faced with continuous evolution in employer-employee relations including: an increasing number and complexity of federal, state, and local regulations; changing workforce dynamics; and challenges attracting and retaining talent. The workplace is rapidly changing as