Barely 18 years old, he already participated at the Salon of Ghent, showing two portraits. Soon afterwards followed his Self-portrait with pipe (1880), painted in somber colours in the Belgian realistic tradition of that time. His Child in an open spot of the forest (1880) already departs from this style and he sets his first steps towards impressionism. Yet soon he would develop his own realistic style, close to impressionism. In 1881 he exhibited for the first time at the Salon in Brussels.
Back in Belgium, he showed about 30 works of his trip at the "Cercle Artistique et Littéraire" in Ghent. It was an instant success, especially The kef smokers, The orange seller and a seascape The strait (setting sun), Tanger (1882).
In April 1883 he exhibited these scenes of everyday Mediterranean life at the salon L'Essor in Brussels before an enthusiast public. It was also around this time that he became befriended with the writer and poet Emile Verhaeren, who would later be portrayed several times by van Rysselberghe.
In September 1883 van Rysselberghe went to Haarlem to study the light in the works of Frans Hals. The accurate rendering of light would continue to occupy his mind. He also met there the American painter William Merritt Chase.
At the second show of Les XX in 1885 Théo van Rysselberghe showed his Arabian phantasia and other images and paintings from his second Moroccan trip, such as Abraham Sicsu (interpreter in Tanger) (1884).
He saw the works of the impressionists Monet and Auguste Renoir at the show of Les XX in 1886. He was deeply impressed. He experimented with this technique, as can be seen in Woman with Japanese album (1886). This impressionist influence became prominent in his paintings Madame Picard in her Loge (1886) and Madame Oscar Ghysbrecht (1886) (painted in a palette of bright colours). In 1887 he painted some impressionist seascapes at the Belgian coast : Het Zwin at high tide (1887)
Because of his growing ties with the Parisian art scene, Octave Maus sent him as a talent scout to Paris to look out for new talent for the next exhibitions of Les XX.
Théo van Rysselberghe abandoned realism and became an adept of pointillism. This brought him sometimes in heavy conflict with James Ensor. In 1887 van Rysselberghe already experimented with this style, as can be seen in his Madame Oscar Ghysbrecht (1887) and Madame Edmond Picard (1887). While staying in summer 1887 a few weeks with Eugène Boch (brother of Anna Boch) in Batignolles, near Paris, he met several painters from the Parisian scene such as Sisley, Signac, Degas and especially Henri de Toulouse-Lautrec. He appreciated especially the talent of Toulouse-Lautrec. His portrait Pierre-Marie Olin (1887) closely resembles the style of Toulouse-Lautrec of that time. He managed to invite several of them, including Signac, Forain, and Toulouse-Lautrec to the next exhibition of Les XX.
He now turned to portraiture, resulting in a series of remarkable neo-impressionist portraits.
His famous portrait of Alice Sèthe (1888) in blue and gold would become a turning point in his life. This time he used merely points in the portrait. She would later marry the sculptor Paul Dubois. Her sister, Maria Sèthe, also a model of van Rysselberghe, would marry the renowned Art Nouveau architect Henry Van de Velde. In that period he made many Neo-impressionistic portraits, such as the portrait of his wife Maria and their daughter Elisabeth. He had married Marie Monnom in 1889. They went on their honeymoon to the south of England and then to Brittany. This would also result in a number of Neo-impressionistic paintings. In Paris he had a meeting with Theo Van Gogh and managed thus to invite Vincent Van Gogh to the next exhibition in Brussels. That is where Van Gogh sold Vigne Rouge in Montmajour to Anna Boch, the only painting he ever sold.
Apart from the portraits, he also painted in this period many landscapes and seascapes : "Dunes in Cadzand" (1893), "The rainbow" (1894).
In the 1895 he made long journeys to Athens and Constantinople, Hungary, Romania, Moscow and Saint Petersburg in order to make posters for the "Compagnie des Wagons-lits". One famous work is the poster "Royal Palace Hotel, Ostende" (1899).
In 1897, van Rysselberghe moved to Paris. Along with Paul Signac, Maximilien Luce, Aristide Delannoy, Alexandre Steinlen, Camille Pissarro, Van Dongen, George Willaume, etc., he contributed to the anarchist magazine Temps Nouveaux.
In the final years of the 1890s, Théo van Rysselberghe had reached the climax of his Neo-impressionist technique. Slowly he abandoned the use of dots in his portraits and landscapes and began applying somewhat broader strokes : The hippodrome at Boulogne-sur-Mer (1900) and the group portrait Summer afternoon (1900), Young women on the beach (1901), Young girl with straw bonnet (1901), and The Reading (1903) (with the contrast between red and blue colours).
After all his years as talent scout for Octave Maus, van Rysselberghe made the mistake of his life: he didn't recognize the talent of the young Pablo Picasso (who was in his Blue Period at that time). He found his works "ugly and uninteresting".
After 1903, his pointillist technique, which he had used for so many years, became more relaxed and after 1910 he abandoned it completely. His strokes had become longer and he used more often vivid colours and more intense contrasts, or softened hues. He had become a master in applying light and heat in his paintings. His Olive trees near Nice (1905) remind us of the technique used by Vincent Van Gogh. These longer strokes in red and mauve become prominent in his Bathing ladies under the pine trees at Cavalière (1905)
After some prospecting, touring on his bike, together with his friend Henri-Edmond Cross, of the Mediterranean coast between Hyères and Monaco, he found an interesting spot in Saint-Clair (where Cross already resided). His brother (and neighbour), the architect Octave van Rysselberghe, built him there a residence in 1911. He retired now to the Côte d'Azur and became more and more detached from the Brussels art scene.
Here he continued painting, mostly landscapes of the Mediterranean coast, portraits (of his wife and daughter, and of his brother Octave) and decorative murals. From 1905 on, the female nude becomes prominent in his monumental paintings : "After the bath" (1910). His painting The vines in October (1912) is painted in lively colours of red, green and blue. One of his last works was Girl in a bath tub (1925).
At the end of his life, he also turned to portrait sculpture, such as the Head of André Gide.
Much of the works of one of the greatest neo-impressionist painters still remain in private collections. They can only rarely be seen. One recent occasion was the retrospective Théo van Rysselberghe in Brussels and later in The Hague between February and September 2006. In November 2005, his work Port Cette (1892) fetched a record 2.6m € at an auction in New York.