Short answer. Yes. Article 1807 makes every partner account to the partnership for any benefit, and hold as trustee any profits he derived — without the others' consent — from any transaction connected with the formation, conduct or liquidation of the partnership, or from any use of its property. A secret profit is not his to keep; it belongs to the firm.
What the law says
Every partner must account to the partnership for any benefit, and hold as trustee for it any profits derived by him without the consent of the other partners
Civil Code, Article 1807 — Partner as Trustee of Benefits. Read the full provision →
You hold a secret profit as trustee
Article 1807 states the core fiduciary duty of a partner in blunt terms. Every partner must account to the partnership for any benefit, and hold as trustee for it any profits derived by him without the consent of the other partners from the firm's affairs or property. Two ideas sit inside that. First, a partner must account — disclose and pay over — any benefit he took. Second, until he does, he holds it as a trustee: the profit is not his to enjoy, it is the partnership's, and he keeps it only on the firm's behalf.
What the duty reaches
The article draws the net widely. It catches any profit from any transaction connected with the formation, conduct, or liquidation of the partnership or from any use by him of its property. So a kickback taken from a supplier the firm deals with, a commission pocketed on a sale made for the partnership, a personal profit from using the firm's assets or opportunities, and secret gains skimmed while winding the business up all fall within it. The key is the connection to the partnership: if the benefit came to the partner because of the firm — its dealings, its property, its opportunities — it is caught, whatever form it took.
Consent is the dividing line
Everything turns on consent. The duty bites on profits taken without the consent of the other partners; a benefit the partners knew about and agreed to is not a breach. So a partner who wants to do business on the side that touches the firm, or to take a fee connected with its dealings, has a simple and honest path: disclose it fully and get the others' agreement first. What he cannot do is help himself and stay silent, then argue after the fact that the firm was not harmed or that he would have been allowed.
If a partner has profited secretly
If you suspect a co-partner has taken a hidden benefit, the remedy this article gives is an accounting: he must disclose the profit and hand it over to the partnership, because he held it as trustee all along. Gather what points to it — the transaction, the connection to the firm, the absence of any consent by the partners. And for your own protection as a partner, the lesson runs the other way too: anything you take that is connected to the firm should be on the table and agreed, not tucked away, because a benefit kept quiet is one you may have to give back.