The registration symbol "®" (namely, the letter R enclosed in a circle) is intended to give notice that a trademark has been federally registered in the United States Patent and Trademark Office ("USPTO"). See 15 U.S.C. § 1111 (Lanham Act § 29). 

Use of the registration symbol is optional, but it provides constructive notice of trademark rights that can enable the trademark owner to recover damages for past infringement without having to prove that the infringer had actual notice of the registration. See 15 U.S.C. § 1117(a) (Lanham Act § 35(a)).

Misuse of the trademark registration symbol, however, could adversely affect the ability to register the mark with the USPTO or enforce the trademark against others, if an intent to deceive can be established. 

“The improper use of the registration symbol by an applicant will defeat applicant's right to registration only in those cases where it is conclusively established that the misuse of the symbol was occasioned by an intent to deceive the purchasing public or others in the trade into believing that the mark was registered.” Copeland's Enters. Inc. v. CNV Inc., 945 F.2d 1563, 20 USPQ2d 1295, 1298 (Fed. Cir. 1991) (quoting Johnson Controls, Inc. v. Concorde Battery Corp., 228 USPQ 39, 44 (TTAB 1985)).

“The use of the registration symbol for goods other than those specified in the registration may constitute an erroneous use, but to show that such use constitutes ‘unclean hands,' it was incumbent upon applicant to show an intent to mislead or deceive in fact.” Penn Diaries, Inc. v. Pa. Agric. Coop. Mktg. Ass'n, 200 USPQ 462, 464 (TTAB 1978). See also Fort Howard Paper Co. v. Federated Foods, Inc., 189 USPQ 310, 314 (TTAB 1975) (“improper use of a registration notice by an applicant is actionable only when it can be conclusively established that such use was occasioned by an intent, actual or implied, to deceive the purchasing public or even others in the trade into believing that the mark is in fact a registered mark entitled to all the presumptions under Section 7(b) of the Statute.”), aff'd, 544 F.2d 1098, 192 USPQ 24, 27 (CCPA 1976).

If a trademark owner quickly corrects a mistake in using the registration symbol, then that may mitigate against finding an intention to deceive the public. 

For unregistered marks, including where there is a pending trademark application but no registration, an informal trademark notice, such as "TM" (for trademark) or "SM" (for service mark) may be used instead to indicate a claim for common law trademark rights.