The Marschallin's bedroom
Rising reluctantly from the bed where they have spent the night together, the Marschallin and Octavian are breakfasting when raised voices outside make the Marscahallin fear that her husband, supposedly far away hunting, has suddenly returned.
Octavian hides, dresses himself in female garments and presents himself to the Marschallin as her maid from the country when the intruder, the Marschallin's country cousin, Baron Ochs von Lerchenau, bursts in. Octavian's attempts to creep out are thwarted by the baron, who attempts to make an assignation with "Mariandel" at the same time as he explains his errand. He has come about his forthcoming marriage to Sophie Faninal, who is young and beautiful and has a rich father, who, however, has only recently been ennobled; but Ochs is satisfied that he has enough noble blood for two.
He has come to ask the Marschallin to choose a young relative to take a silver rose to Sophie as a token of betrothal, and for a recommendation to her notary, who is expected at her levee. His attempts to grab Mariandel cause the Marschallin to remark that his engagement has not cramped his style, and he boasts that he has no intention of curtailing his amorous exploits, adding that he keeps one of his bastard sons as his body servant. When the Marschallin suggests Octavian as the rose-bearer and shows him a picture, he is struck by the resemblance to Mariandel and the Marschallin has to pretend that "she" is Octavian's bastard sister, to forestall Ochs' curiosity when he meets Octavian.
At the levee, the Marschallin - among other things - has her hair done, gives alms to three noble orphans, refuses a gossip sheet offered by Valzacchi, an Italian intriguer, and listens to an Italian tenor, whose song is interrupted by the baron's arguments with the notary over the marriage settlement.
Complaining that her hairdresser has made her look old, the Marschallin sends everyone away. Before leaving, Ochs manages to ask Valzacchi and his accomplice Annina to arrange for him to meet Mariandel, a commission they accept eagerly despite the fact that they have no idea who he means. Leopold, the body servant, brings the silver rose. The Marschallin reflects on the complacency of Ochs, about to marry a young girl with money, but confident he is the one conferring the favor. She remembers how she was taken straight from the convent and thrust into marriage.
Octavian, returning in his own clothes, finds her mood changed, as she tells him how she feels time passing and herself growing old. When he tries to cheer her up, she says she knows that one day he will leave her for someone younger and more beautiful. Sweeping aside his protestations, she sends him away, but then is sorry because they have parted without a kiss. She sends servants ro recall him, but he is already out of sight. She calls for her little black servant Mahomet and gives him the silver rose to take to Octavian.
The salon in Faninal's house
Sophie tries to remain calm amid the excited comments of her duenna and the frenzied household preparations. Octavian arrives and presents the rose to Sophie. Both remain transfixed with eyes only for one another. They make polite conversation until Ochs is presented by Faninal. Sophie is repelled by his pockmarked complexion and coarse blandishments and Octavian is barely able to contain his anger. Faninal and Ochs withdraw to sign the contract, but Octavian and Sophie are prevented from coming to an understanding by the presence of her duenna.
She, however, is called away to quell the disturbance caused by Ochs' drunken servants terrorising the maids, and Sophie confides to Octavian that she will not marry Ochs. They confess their love, but as they kiss, Annina and Valzacchi creep up, grab them and call for the baron. He is unconcerned, having given Octavian permission to warm Sophie up, and even when Octavian tells him that Sophie does not like him, he only remarks that she will soon learn to; so Octavian draws his sword and insults him, forcing to draw his own sword. Ochs is lightly wounded in the arm, but makes a great fuss. Faninal tries to soothe him and threatens Sophie that she will either marry him (alive or dead) or spend her life in a convent. Octavian is obliged to leave.
Bandaged and supplied with wine, Ochs becomes more cheerful. Annina brings him a note from Mariandel, offering an assignation for the next night. He ignores her pointedly outstretched hand.
A private room in an inn
With the assistance of Valzacchi and Annina, Octavian, wearing his Mariandel costume over his ordinary clothes, prepares the scene for the discomfiture of Ochs.
Ochs arrives, and his advances to Mariandel, interspersed with moments of aversion as the face reminds him of his encounter with Octavian, are interrupted by the appearance of heads from trapdoors and windows, culminating in the entry of Annina in disguise, representing herself as his deserted wife and accompanied by children screaming "Papa!" Ochs, who has lost his wig, loses his head and calls for the police; but when the police commissioner arrives, he is unable to provide an acceptable explanation for his dubious situation.
He claims that Mariandel is his fiancee, Sophie Faninal, but her father (sent for by Octavian) arrives at that moment. Faninal identifies Ochs as his prospective son-in-law, but disclaims Mariandel. Sophie leads her father outside and the Marschallin (summoned by a message from Ochs) appears. Sophie brings a message from her father repudiating the marriage, and Octavian, now in his own clothes, nervously tries to explain Sophie's identity to the Marschallin.
The Marschallin dismisses the commissioner with the explanation that it has all been a farce, then sends Ochs packing, firmly reminding him that he is a gentleman, when the realisation that Octavian and Mariandel are one and the same sets him speculating. He leaves, pursued by Annina and screaming children and the innkeepr and others demanding payment. Left between Sophie, who fears that their love might be part of the farce, and the Marschallin, Octavian is undecided, but finally goes to Sophie when the Marschallin advises him to follow his heart. Reassuring the nervous Sophie, the Marschallin, true to her vow to give up Octavian gracefully when the time comes, withdraws and leaves them together.