In mergers and acquisitions, a mandatory offer, also called a mandatory bid in some jurisdictions, is an offer made by one company (the "acquiring company" or "bidder") to purchase some or all outstanding shares of another company (the "target"), as required by securities laws and regulations or stock exchange rules governing corporate takeovers. Most countries, with the notable exception of the United States, have provisions requiring mandatory offers.

Typically, a mandatory offer must be made when the acquiring company exceeds a certain shareholding threshold in the target, or gains actual control of the target. Most countries, with the notable exception of the United States, have such a requirement. The purpose of mandatory offer regulations is to protect minority shareholders in situations where control of the target is being transferred, and in particular to discourage acquisitions driven by private benefits of control by requiring that a premium be paid for such control.

Thresholds for mandatory offers vary widely between countries. A 2006 World Bank survey of the laws of 50 countries found thresholds ranging from 15% (India; became 20% in 2011) to 67% (Finland); the author's literature review did not find any study of the optimal level for the threshold. Thirty percent is a fairly common threshold, found in the City Code on Takeovers and Mergers followed in the United Kingdom as well as the laws of several other European countries. Depending on the ownership structure of the target, any given threshold short of absolute majority may in practice result in an acquirer being obliged to make an offer even when the acquirer has not yet established a controlling interest in the target, and even when another shareholder besides the acquirer retains the majority of shares; conversely, if the remaining shares not held by the acquirer are highly dispersed, actual control of the target could be attained without crossing the mandatory offer threshold. Takeover legislation in various European countries in the 1990s, for example in Austria, thus attempted to use de facto control rather than a specific threshold as the trigger for a mandatory offer; however, this presented enforcement difficulties, given that the threshold for control varied not only between target corporations but over time in the same target, and regulations later provided for a presumption of control upon achieving a specific threshold.

A mandatory offer rule is distinct from tag-along rights, which give minority shareholders the right to join in any sale by the majority shareholder: the former is an obligation imposed on the acquirer by laws and regulations, while the latter may be provided voluntarily by the majority shareholder of the target to minority shareholders through unilateral announcement or in a shareholders' agreement or similar private contract.

Brazilian corporate law provided for a mandatory offer rule prior to 1997. It was repealed that year, but then partially reinstated in 2000 due to pressure from institutional investors. A major transaction concluded prior to the rule's reinstatement was the Brazilian government's sale of its 66.7% of voting shares in Banco Banespa to Banco Santander, in which Banco Santander's tender offer covered only the government's state and excluded the minority shareholders.

In the United States, the Williams Act of 1968, which regulates tender offers, does not contain any provisions requiring mandatory offers, due to concerns that such provisions could increase transaction costs in mergers and acquisitions.

A mandatory bid rule has been applicable in some form since 1993 to companies listed on the Shanghai Stock Exchange and Shenzhen Stock Exchange, although the China Securities Regulatory Commission (CSRC) is empowered to grant exemptions, and in practise acquirers can also structure their transactions to avoid triggering the rule in many cases. The first version of the rule was adapted from the similar rule in Hong Kong, as at the time many state-owned enterprises of China had already listed on the Hong Kong Stock Exchange, and the regulations' drafters sought advice from Hong Kong experts. The rule provided that upon acquiring 30% of the outstanding common stock of the target, the acquirer must make a bid for all remaining outstanding shares within 45 working days, at a price which at least matches the highest price paid by the acquirer in the past year for the shares it already holds, as well as the average market price of the shares within the last 30 days. That rule remained in effect after the passage of the 1998 Securities Law [zh], though the 1999 law relaxed some related reporting requirements. 2002 regulations maintained the 30% threshold, but provided that the mandatory bid only had to be made if the acquirer intended to acquire more shares, and permitted the CSRC to grant exceptions to the obligation to bid. In 2006, both the Securities Law and the 2002 regulations were revised; Article 88 of the revised law left the 30% threshold intact, but weakened the bid requirement from the earlier mandatory bid for all outstanding shares to a bid for at least 5% of the outstanding shares.

In Hong Kong, mandatory offers have been governed since 1975 by Rule 26 of the Code on Takeovers and Mergers, issued by the Securities and Futures Commission. The code does not have force of law. However, compliance with the code is required by Listing Rule 13.23 of the Hong Kong Stock Exchange.