Few people can readily forgive a spouse’s romantic infidelity. But millions of Americans are tolerating a marriage transgression that’s just as divisive. Financial infidelity is undermining more marriages than we realize.

According to a new poll commissioned by CreditCards.com, nearly 13 million Americans could be hiding secret bank accounts or credit cards from their partners.

This mirrors findings of a recent survey performed by the National Endowment for Financial Education (NEFE), which revealed that two in five Americans admitted to having committed financial infidelity in their relationships. This financial cheating included hiding extra sources of income, opening covert bank accounts, making secret purchases and lying about debt.

Yet, as with any type of infidelity, these financial transgressions can cause serious harm to a marriage. According to the NEFE survey, 75 percent of respondents claim their financial deception affected their relationship.

“Financial infidelity hurts regardless of its scale. Hiding or lying about small amounts of money can damage a relationship just as effectively as a high-dollar deceit. In fact, in all cases of this deception, people affected say it impacted their relationship in some way–almost always negatively. It causes arguments, erosion of trust, separation or even divorce,” explained Ted Beck, president and CEO of NEFE, in a Parade magazine interview.

So what can you do to protect your marriage from the corrupting consequences of financial infidelity?

Begin by putting all your cards on the table.

Yes, it may be tough, even embarrassing, to reveal all your financial warts, but a solid marriage is built on trust and transparency. It’s imperative partners reveal all of their personal financial information, including debts, salaries and even their not-so-above board financial actions like hoarding secret cash or making secret purchases. Only by being completely honest, can you begin to build the financial intimacy required for a strong marriage.

Develop a game plan.

Next, it’s important to develop a game plan for how to tackle financial issues in the future. Perhaps you create a joint account for all the household’s major expenses while also maintaining separate accounts for personal purchases. Maybe you agree to consult each other for any purchase over a certain dollar threshold, say $300 for instance. Whatever you decide, it is crucial to develop financial ground rules in your marriage, and, just as importantly, stick to them.

Continue to communicate.

Set aside time each month to continue to communicate about your money. Discuss what’s working and what’s not working. Make sure to talk about any important upcoming financial decisions you will be facing as a couple.

Financial infidelity can ruin a marriage but you don’t have to succumb to the temptation. With open and honest communication, couples can build a rock strong financial marriage.

BMWK, what do you do to keep financial infidelity from creeping into your marriage?