diff --git a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -20,7 +20,7 @@
 include(KDEFrameworkCompilerSettings)
 include(KDECMakeSettings)
 
-set(REQUIRED_QT_VERSION 5.3.0)
+set(REQUIRED_QT_VERSION 5.5.0)
 find_package(Qt5 ${REQUIRED_QT_VERSION} CONFIG REQUIRED Widgets DBus)
 
 set(KF5_VERSION "5.18.0") # handled by release scripts
diff --git a/src/platformtheme/kdeplatformfiledialoghelper.cpp b/src/platformtheme/kdeplatformfiledialoghelper.cpp
--- a/src/platformtheme/kdeplatformfiledialoghelper.cpp
+++ b/src/platformtheme/kdeplatformfiledialoghelper.cpp
@@ -221,13 +221,7 @@
 {
     if (options()->testOption(QFileDialogOptions::ShowDirsOnly)) {
         m_dialog->deleteLater();
-        m_dialog = new KDirSelectDialog(
-#if QT_VERSION >= QT_VERSION_CHECK(5,5,0)
-            options()->initialDirectory()
-#else
-            m_dialog->directory()
-#endif
-        );
+        m_dialog = new KDirSelectDialog(options()->initialDirectory());
         connect(m_dialog, SIGNAL(accepted()), SIGNAL(accept()));
         connect(m_dialog, SIGNAL(rejected()), SIGNAL(reject()));
         if (!options()->windowTitle().isEmpty())
@@ -241,9 +235,7 @@
         } else {
             dialog->setWindowTitle(options()->windowTitle());
         }
-#if QT_VERSION >= QT_VERSION_CHECK(5,5,0)
         setDirectory(options()->initialDirectory());
-#endif
         //dialog->setViewMode(options()->viewMode()); // don't override our options, fixes remembering the chosen view mode and sizes!
         dialog->setFileMode(options()->fileMode());
 
diff --git a/src/platformtheme/kdeplatformsystemtrayicon.h b/src/platformtheme/kdeplatformsystemtrayicon.h
--- a/src/platformtheme/kdeplatformsystemtrayicon.h
+++ b/src/platformtheme/kdeplatformsystemtrayicon.h
@@ -28,7 +28,6 @@
 class QAction;
 class QMenu;
 
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 3, 0))
 class SystemTrayMenu : public QPlatformMenu
 {
     Q_OBJECT
@@ -76,19 +75,14 @@
     void setText(const QString &text) Q_DECL_OVERRIDE;
     void setVisible(bool isVisible) Q_DECL_OVERRIDE;
     quintptr tag() const Q_DECL_OVERRIDE;
-    void setIconSize(int size)
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 4, 0))
-    Q_DECL_OVERRIDE
-#endif
-    ;
+    void setIconSize(int size) Q_DECL_OVERRIDE;
 
     QAction *action() const;
 
 private:
     quintptr m_tag;
     QAction *m_action;
 };
-#endif
 
 class KDEPlatformSystemTrayIcon : public QPlatformSystemTrayIcon
 {
@@ -108,9 +102,7 @@
     bool isSystemTrayAvailable() const Q_DECL_OVERRIDE;
     bool supportsMessages() const Q_DECL_OVERRIDE;
 
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 3, 0))
     QPlatformMenu *createMenu() const Q_DECL_OVERRIDE;
-#endif
 
 private:
     KStatusNotifierItem *m_sni;
diff --git a/src/platformtheme/kdeplatformsystemtrayicon.cpp b/src/platformtheme/kdeplatformsystemtrayicon.cpp
--- a/src/platformtheme/kdeplatformsystemtrayicon.cpp
+++ b/src/platformtheme/kdeplatformsystemtrayicon.cpp
@@ -26,7 +26,6 @@
 #include <QApplication>
 #include <QDBusInterface>
 
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 3, 0))
 SystemTrayMenu::SystemTrayMenu()
     : QPlatformMenu()
     , m_tag(0)
@@ -252,7 +251,6 @@
 {
     return m_action;
 }
-#endif
 
 KDEPlatformSystemTrayIcon::KDEPlatformSystemTrayIcon()
     : QPlatformSystemTrayIcon()
@@ -312,24 +310,18 @@
 
 void KDEPlatformSystemTrayIcon::updateMenu(QPlatformMenu *menu)
 {
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 3, 0))
     if (!m_sni) {
         return;
     }
     if (SystemTrayMenu *ourMenu = qobject_cast<SystemTrayMenu*>(menu)) {
         m_sni->setContextMenu(ourMenu->menu());
     }
-#else
-    Q_UNUSED(menu)
-#endif
 }
 
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 3, 0))
 QPlatformMenu *KDEPlatformSystemTrayIcon::createMenu() const
 {
     return new SystemTrayMenu();
 }
-#endif
 
 QRect KDEPlatformSystemTrayIcon::geometry() const
 {
diff --git a/src/platformtheme/kdeplatformtheme.h b/src/platformtheme/kdeplatformtheme.h
--- a/src/platformtheme/kdeplatformtheme.h
+++ b/src/platformtheme/kdeplatformtheme.h
@@ -46,9 +46,7 @@
     QPlatformDialogHelper *createPlatformDialogHelper(DialogType type) const Q_DECL_OVERRIDE;
     bool usePlatformNativeDialog(DialogType type) const Q_DECL_OVERRIDE;
 
-#if QT_VERSION >= QT_VERSION_CHECK(5, 3, 0)
     QString standardButtonText(int button) const Q_DECL_OVERRIDE;
-#endif
 
     QPlatformSystemTrayIcon *createPlatformSystemTrayIcon() const Q_DECL_OVERRIDE;
 
diff --git a/src/platformtheme/kdeplatformtheme.cpp b/src/platformtheme/kdeplatformtheme.cpp
--- a/src/platformtheme/kdeplatformtheme.cpp
+++ b/src/platformtheme/kdeplatformtheme.cpp
@@ -212,7 +212,6 @@
     return type == QPlatformTheme::FileDialog;
 }
 
-#if QT_VERSION >= QT_VERSION_CHECK(5, 3, 0)
 QString KdePlatformTheme::standardButtonText(int button) const
 {
     switch (static_cast<QPlatformDialogHelper::StandardButton>(button)) {
@@ -260,7 +259,6 @@
         return QPlatformTheme::defaultStandardButtonText(button);
     }
 }
-#endif
 
 QPlatformDialogHelper *KdePlatformTheme::createPlatformDialogHelper(QPlatformTheme::DialogType type) const
 {
diff --git a/src/platformtheme/khintssettings.cpp b/src/platformtheme/khintssettings.cpp
--- a/src/platformtheme/khintssettings.cpp
+++ b/src/platformtheme/khintssettings.cpp
@@ -106,9 +106,7 @@
     m_hints[QPlatformTheme::UiEffects] = cg.readEntry("GraphicEffectsLevel", 0) != 0 ? QPlatformTheme::GeneralUiEffect : 0;
     m_hints[QPlatformTheme::IconPixmapSizes] = QVariant::fromValue(QList<int>() << 512 << 256 << 128 << 64 << 32 << 22 << 16 << 8);
 
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 5, 0))
     m_hints[QPlatformTheme::WheelScrollLines] = cg.readEntry("WheelScrollLines", 3);
-#endif
     if (qobject_cast<QApplication *>(QCoreApplication::instance())) {
         QApplication::setWheelScrollLines(cg.readEntry("WheelScrollLines", 3));
     }
@@ -331,10 +329,8 @@
     bool showIcons = cg.readEntry("ShowIconsInMenuItems", !QApplication::testAttribute(Qt::AA_DontShowIconsInMenus));
     QCoreApplication::setAttribute(Qt::AA_DontShowIconsInMenus, !showIcons);
 
-#if (QT_VERSION >= QT_VERSION_CHECK(5, 5, 0))
     int wheelScrollLines = cg.readEntry("WheelScrollLines", 3);
     m_hints[QPlatformTheme::WheelScrollLines] = wheelScrollLines;
-#endif
     QApplication *app = qobject_cast<QApplication *>(QCoreApplication::instance());
     if (app) {
         QApplication::setWheelScrollLines(cg.readEntry("WheelScrollLines", 3));
diff --git a/src/platformtheme/main.cpp b/src/platformtheme/main.cpp
--- a/src/platformtheme/main.cpp
+++ b/src/platformtheme/main.cpp
@@ -22,16 +22,6 @@
 
 #include "kdeplatformtheme.h"
 
-#include <config-platformtheme.h>
-#if (QT_VERSION < QT_VERSION_CHECK(5, 3, 1))
-#if HAVE_X11
-#include <QCoreApplication>
-#include <QAbstractEventDispatcher>
-#include <QX11Info>
-#include <xcb/xcb.h>
-#endif
-#endif
-
 class KdePlatformThemePlugin : public QPlatformThemePlugin
 {
     Q_OBJECT
@@ -44,37 +34,8 @@
     {
         Q_UNUSED(key)
         Q_UNUSED(paramList)
-#if (QT_VERSION < QT_VERSION_CHECK(5, 3, 1))
-        // Must be done after we have an event-dispatcher. By posting a method invocation
-        // we are sure that by the time the method is called we have an event-dispatcher.
-        QMetaObject::invokeMethod(this, "setupXcbFlush", Qt::QueuedConnection);
-#endif
         return new KdePlatformTheme;
     }
-
-public Q_SLOTS:
-    void setupXcbFlush();
 };
 
-void KdePlatformThemePlugin::setupXcbFlush()
-{
-#if (QT_VERSION < QT_VERSION_CHECK(5, 3, 1))
-#if HAVE_X11
-    // this is a workaround for BUG 334858
-    // it ensures that the xcb connection gets flushed before the EventDispatcher
-    // is going to block. Qt does not guarantee this in all cases.
-    // For Qt this issue is addressed in https://codereview.qt-project.org/85654
-    // TODO: remove again once we depend on a Qt version with the patch.
-    if (!QX11Info::isPlatformX11() || qstrcmp(qVersion(), "5.3.1") >= 0) {
-        return;
-    }
-    connect(QCoreApplication::eventDispatcher(), &QAbstractEventDispatcher::aboutToBlock,
-        []() {
-            xcb_flush(QX11Info::connection());
-        }
-    );
-#endif
-#endif
-}
-
 #include "main.moc"