Whether we can keep money we've found and what we can do with it are ethical as well as legal questions. Even though cash is not marked with the owner's identity—like a check or savings bond—it's a piece of property that originally belonged to someone other than the finder. Cash you find is not legally yours—it's still the owner's money. Your state or local law will likely have something to say regarding what you need to do if you find property or cash.
If you find money, especially a significant amount, you should check your local laws or contact an attorney or the police. Many communities have local laws or ordinances governing what someone must do if they find cash and don't know who it belongs to. In some instances, state law will apply.
These laws usually require that a person who finds money, especially larger amounts (for example, $100 or more), turn it over to the local police. Depending on the jurisdiction, the law might authorize the police to return the money to the finder if no one claims it after a certain period of time (such as three to six months). Some laws direct police to deposit the cash in the jurisdiction's coffers if no one claims it.
If the law requires you to turn over found money to the police and you don't, or if you don't make reasonable efforts to identify the owner, you could be charged with larceny or theft.
Most states make it a theft crime to keep lost, mislaid, or misdelivered property without using reasonable measures to find the owner. Even though you did not steal the money by taking it directly from its owner, you are holding the money belonging to another and not trying to return it.
Reasonable measures likely depend on how you found the money and how much. If you find a $20 bill on the street, it's likely reasonable to ask people close by if they dropped any cash. If you find a wallet with money in it, you should check for ID or credit cards with identification information. You can call the customer service number on a credit card to see if the credit card company can locate the owner. Or see if you can locate a phone number for the person's address. If you find the wallet on a restaurant floor or other public place, you can bring it to the closest store in hopes that the wallet owner retraces their steps. You can also contact the non-emergency line for police and ask for assistance in determining what to do, especially if you find large amounts of cash.
In most states, the penalties for theft of lost property will be based on its value. The larger the amount is, the stiffer the penalty will be. For instance, if you find more than $950 in California and keep the money "without first making reasonable and just efforts to find the owner," you could be charged with felony theft. For amounts of $950 or less, the penalty in California is a misdemeanor. (Cal. Penal Code §§ 485, 487.)
Misdemeanor or felony theft. Every state theft laws use different dollar amounts to divide misdemeanor from felony crimes. Most states penalize misdemeanor theft with possible jail time of up to one year, plus fines. Felony theft penalties can mean prison time of a year or more. Check your state's theft laws for more information.
Receiving stolen property. If you know the cash was stolen and you keep it, you could be looking at penalties for receiving stolen property. (Cal. Penal Code § 496.) Depending on the state, this offense might be punished the same as, or more harshly than, theft or larceny.
Especially if you find a large amount of money, it probably is best to contact the police or a criminal defense attorney who can verify local laws regarding lost and found cash and advise you on how to proceed.